University of Wollongong University of Wollongong

Research Online Research Online

Faculty of Engineering and Information Sciences - Papers: Part B

Faculty of Engineering and Information Sciences

2018

Application of solar energy in water treatment processes: A review Application of solar energy in water treatment processes: A review

Ying Zhang University of Wollongong, yz393@uowmail.edu.au

Muttucumaru Sivakumar University of Wollongong, siva@uow.edu.au

Shu-qing Yang University of Wollongong, shuqing@uow.edu.au

Keith Enever University of Wollongong, kenever@uow.edu.au

Mohammad Ramezanianpour Ara Institute of Canterbury, mr417@uowmail.edu.au

Follow this and additional works at: https://ro.uow.edu.au/eispapers1

Part of the Engineering Commons, and the Science and Technology Studies Commons

Recommended Citation Recommended Citation Zhang, Ying; Sivakumar, Muttucumaru; Yang, Shu-qing; Enever, Keith; and Ramezanianpour, Mohammad, "Application of solar energy in water treatment processes: A review" (2018). Faculty of Engineering and Information Sciences - Papers: Part B. 841. https://ro.uow.edu.au/eispapers1/841

Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au

Application of solar energy in water treatment processes: A review Application of solar energy in water treatment processes: A review

Abstract Abstract The utilization of solar energy to drive water treatment processes is a potential sustainable solution to the world's water scarcity issue. In recent years, significant efforts have been devoted to developing and testing innovative solar based water treatment technologies, which are comprehensively reviewed in this paper. Recent developments and applications of seven major solar desalination technologies, solar photocatalysis process and solar disinfection are investigated. Potential integration of solar technologies and desalination processes are summarized. By collecting and analysing performance data from recent studies, the status of productivity, energy consumption and water production costs of different technologies is critically reviewed. The real world applicability as well as technical and economic feasibility is also evaluated. Presently, most of the solar water treatment processes are still under development with limited real applications. Economic competitiveness is among the major reasons that affect the scaling up and commercialization. It is revealed that the reported water costs of small to

medium scale solar desalination plants are in the range of US$0.2-22/m3 , much higher than conventional

fossil fuel based plants. However, the estimated low water costs (US$0.9-2.2/m3 ) for large scale solar based plants indicate that solar based alternatives will become potentially viable in the near future.

Disciplines Disciplines Engineering | Science and Technology Studies

Publication Details Publication Details Zhang, Y., Sivakumar, M., Yang, S., Enever, K. & Ramezanianpour, M. (2018). Application of solar energy in water treatment processes: A review. Desalination, 428 116-145.

This journal article is available at Research Online: https://ro.uow.edu.au/eispapers1/841

1

Application of solar energy in water treatment processes: a review

Ying Zhang1, Muttucumaru Sivakumar1,*, Shuqing Yang1, Keith Enever1, Mohammad

Ramezanianpour2

1 School of Civil, Mining and Environmental Engineering, Faculty of Engineering and Information

Sciences, University of Wollongong, NSW, Australia

2 Department of Engineering and Architectural Studies, Ara Institute of Canterbury, Christchurch,

New Zealand

Contents

Abstract ............................................................................................................................................................................. 2

Key words ......................................................................................................................................................................... 2

1. Introduction ............................................................................................................................................................... 2

2. Solar desalination technologies ..................................................................................................................................... 3

2.1 Direct solar desalination--solar still ........................................................................................................................ 4

2.2 Indirect solar desalination ..................................................................................................................................... 10

2.2.1 Solar humidification-dehumidification .......................................................................................................... 10

2.2.2 Solar-powered MSF ....................................................................................................................................... 16

2.2.3 Solar-powered MED ...................................................................................................................................... 21

2.2.4 Solar-powered MD ......................................................................................................................................... 30

2.2.5 Solar-powered RO .......................................................................................................................................... 37

2.2.6 Solar-powered ED .......................................................................................................................................... 42

2.3 Summary of solar desalination technologies......................................................................................................... 45

3. Solar photocatalysis and disinfection .......................................................................................................................... 47

3.1 Solar water photocatalytic application .................................................................................................................. 47

3.2 Solar Disinfection of water (SODIS) .................................................................................................................... 52

4. Conclusions ................................................................................................................................................................. 53

Acknowledgement .......................................................................................................................................................... 54

References ....................................................................................................................................................................... 54

_____________________________

*Corresponding author. Tel.: +61 2 4221 3055.

E-mail addresses: siva@uow.edu.au (M. Sivakumar), yz393@uowmail.edu.au (Y. Zhang)

2

Abstract

The utilization of solar energy to drive water treatment processes is a potential sustainable solution to the world’s

water scarcity issue. In recent years, significant efforts have been devoted to developing and testing innovative solar

based water treatment technologies, which are comprehensively reviewed in this paper. Recent developments and

applications of seven major solar desalination technologies, solar photocatalysis process and solar disinfection are

investigated. Potential integration of solar technologies and desalination processes are summarized. By collecting and

analysing performance data from recent studies, the status of productivity, energy consumption and water production

costs of different technologies is critically reviewed. The real world applicability as well as technical and economic

feasibility is also evaluated. Presently, most of the solar water treatment processes are still under development with

limited real applications. Economic competitiveness is among the major reasons that affect the scaling up and

commercialization. It is revealed that the reported water costs of small to medium scale solar desalination plants are in

the range of US$0.2~22/m3, much higher than conventional fossil fuel based plants. However, the estimated low water

costs (US$0.9~2.2/m3) for large scale solar based plants indicate that solar based alternatives will become potentially

viable in the near future.

Key words

Solar energy; water treatment; desalination technologies; photocatalysis; SODIS

1. Introduction

Water and energy security are two of the major issues mankind must tackle to achieve the sustainable development of

human society. Water scarcity which is already a major challenge faced by many regions is becoming even worse due

to the increasing water demand brought by rapid population and economic growth in developing countries. Meanwhile,

the discharge of municipal and industrial wastewater effluent without proper treatment that caused serious pollution on

fresh water sources has aggravated the problem. According to the United Nations Environmental Programme (UNEP),

1/3 of world population live in water-stressed countries, while by 2025, 2/3 of world population will face water

scarcity [1]. The scarcity of water strongly limits the socio-economic development of these countries.

In 2012, 13,371 million tons oil equivalent (MTOE) of total primary energy supply were consumed in the world, with

81.7% from fossil fuels (oil 31.4%, natural gas 21.3%, and coal 29.0%) and only a small amount from biofuel and

waste (10%), nuclear (4.8%), hydro (2.4%) and other source (1.1%) [2]. Energy demand will continue to increase over

the coming decades to meet the growing population while associated economic development and a 31% increase in

global energy consumption is foreseen by 2035 [2]. However, global reserve of crude oil, natural gas and coal are

depleting. Many scientists believe that an oil production peak has either occurred already or will be likely to occur in

the coming few years [3]. Global oil consumption rate is expected to decline by 75% by 2050 due to the depletion of

many oil reserves. It is also forecasted that natural gas and coal production will peak within decades of oil peak [3].

Meanwhile, the emission of large amount of greenhouse gases and other air pollutants such as hydrocarbons, nitrogen

3

oxide, sulphur dioxide, etc. by combustion of fossil fuels has caused serious environmental concerns. Clean,

renewable primary energy must be utilized to solve the energy crisis in the near future.

Solar energy is by far the most abundant renewable energy source. It shows the highest technical feasible potential

(about 60TW) among all renewable energy sources [4], which surpassed the total world energy consumption

(13,371MTOE is equal to 17.75TW) in 2012. Although presently solar energy only accounts for a very small fraction

of world energy supply (about 0.5% electricity generation globally) [5], the continuous development of modern solar

energy conversion technologies in the past decades is making solar energy systems less expensive and more efficient.

According to International Energy Agency , solar energy could become the largest electricity source by 2050 [6].

To address water shortage, a variety of non-traditional water sources have been considered for water production for

drinking, industrial, agriculture or other usages, such as seawater/brackish water, treated municipal/industrial

wastewater, contaminated surface or groundwater, etc. However, sustainable water supply cannot be achieved without

considering the energy required in the treatment process. Coincidently, many of the world’s arid and semi-arid regions

which face severe water shortage are generally blessed with abundant solar radiation. This allows the address of water

scarcity with sustainable solar energy. Suitable technologies need to be developed to integrate solar energy into water

treatment processes. Solar desalination technologies, solar photocatalysis technologies and solar disinfection are the

most widely investigated solar based water treatment technologies, which will be discussed in detail in this paper.

Among them, solar desalination technologies have received considerable attention all over the world due to its

applicability to arid or remote regions. Various solar desalination technologies have been examined and reviewed [1,

7-14]. The global applicability and opportunities of solar desalination have been further demonstrated by researchers

[1, 8, 11]. Specially, Adrian et al. [8] identified 30 nations with high applicability and 28 countries with ‘moderate

applicability’ by a newly proposed method. Detailed reviews of the principles and features of different solar

desalination technologies have been provided [9, 10, 13] . Sharon et al. [10] also discussed briefly the advantage and

disadvantages of each technology as well as the problems existing in desalination processes. Special focus of

thermodynamic and thermo-economic analysis of solar desalination systems were presented by Iman et al. [9].

However, only limited application cases were shown in these reviews so that the present status and development of

specific technologies were not clearly shown. A very comprehensive review in solar assisted seawater desalination

was given by Li et al [13]. Nevertheless, latest research and applications were not included in this review since it was

written before 2012. Therefore, in this paper, the current status and progress of different solar water treatment

technologies have been extensively reviewed by summarizing research and applications in recent years. The

economics and applicability are also discussed.

2. Solar desalination technologies

Desalination of seawater and brackish water is well known to be an alternative solution to provide fresh water for

many water-stressed regions. For decades, large commercial desalination plants powered by fossil fuels have been

installed in countries that suffer from water shortage, especially oil-rich countries in Middle East. Solar energy can be

used directly or indirectly to drive desalination plants. In direct solar desalination systems, solar energy is used

directly for the production of distilled water in solar collector, with solar still as the most representative technology;

4

whereas in indirect solar desalination systems, solar energy is harvested either by solar thermal collectors to provide

heat or photovoltaic panels to generate electricity for thermal or membrane desalination technologies such as multi-

effect desalination (MED), multi-stage flash desalination (MSF), membrane distillation (MD) or reverse osmosis (RO).

In the sections below, brief descriptions of fundamentals of different desalination processes are provided in order to

discuss the performance evaluation and operation parameters, as well as recent trends of technologies. Detailed

explanations of those processes can be found in books and review papers [9, 10, 13, 15].

2.1 Direct solar desalination--solar still

Solar still is the most common direct solar desalination technology which is mainly suitable for low capacity water

supply systems in remote areas where construction of pipelines or water delivery by truck is uneconomical and

unreliable [16]. The simplest design of a single basin solar still consists of an airtight, sloping transparent cover which

encloses a black painted basin with saline water (see Fig.1). Water evaporates after being heated up with the absorbed

solar energy by the basin. Condensation occurred in the inner surface of the sloping cover and then distilled water is

collected at the lower end of the cover. Despite its technical simplicity and relatively less maintenance requirement,

solar still is not widely used due to the low productivity per unit installation area, normally 1~5 L/m2 /d for a single

basin still. Consequently large areas of land are required for the installation of solar still.

Fig.1 Schematic diagram of a single slope single basin solar still

Parameters affecting the productivity of a basin type solar still include absorption area, water depth, inlet water

temperature, water-glass cover temperature difference, etc. A comprehensive review has been given by Prakash et al.

[17]. Extensive modifications have been carried out to improve the productivity of solar stills. The objectives of

modifications are basically to enhance water evaporation in the basin, condensation on the cover or to recover latent

heat of evaporation. Fig. 2 shows major approaches in literature to improving still productivity [18-23]. Table 1 is a

list of selected solar still studies and applications. Generally solar still can be classified into two categories: passive

and active solar stills. In active solar stills, additional devices are adopted, including vacuum fan, pump, sun tracking

system and solar collectors. Although the efficiency can be enhanced, however, it also results in increasing costs and

5

complexity of the system. A balance needs to be made between improving the productivity and keeping its simplicity

and economic feasibility in the modification of solar still.

The diversity of modifications again indicates its simplicity and little reliance on high technology. Many locally

available materials can be used for its construction and amelioration. For example, Ahsan et al. investigated a low cost

solar still – triangular solar still (TrSS) with cheap, lightweight locally available materials [24]. The TrSS solar still

consisted of a polythene triangular cover, a frame and a rectangular trough, which was made of polythene film, PVC

pipes and perspex, respectively. The whole experimental set-up cost only $35. It is a good example for low cost solar

still for rural and remote area. Meanwhile, an all-plastic solar still system could result in easier maintenance as the

traditional glass cover which is heavy and vulnerable to damage was replaced by the PE film. Besides the approaches

listed in Fig. 2, some researchers have proposed other novel methods or designs. The addition of nano-particles to the

feed water to enhance solar still performance has been studied recently [25, 26]. Nanofluid was expected to possess

superior heat transfer features compared with normal saline water. Kabeel et al. investigated the effect of nanofluid

on the performance of a single basin solar still [26]. The daily productivity increased by 76% and 116% with or

without additional vacuum fan operation. Instead of modifying the conventional basin type still, Ahsan et al. proposed

and fabricated the novel Tubular Solar Still (TSS) using cheap, lightweight materials [27]. The new TSS was made

from polythene film, carton paper and galvanized iron (GI) pipes/wire. The daily water productivity is 5 L/m2.d with

water production cost estimated as $ 9.56/m3, which is much cheaper than the water costs of most of the other lab

scale solar stills (Table 1).

Despite of the various modifications, the productivity is still quite limited, varying from 1~10 L/m2.d in most cases as

shown in Table 1. The thermal efficiency of the system can be evaluated with GORso (Gain Output Ratio solar) and

SSEC (specific solar energy consumption) which are defined in Table 1. The reported GORso are in the range of

0.28~0.94 while mostly less than 0.8 with the corresponding SSEC in the range of 697~2340 kWh/m3. Most reported

studies about solar stills are on a laboratory scale with less than 10 L/d water production. Very little information can

be found in literature about pilot or real plant in recent years, while only a few plants were reported in the 1980s and

1990s. The water costs estimated in different studies varies from $6~143/m3 with most up to $30/m3. Ayoub et al.

estimated the water production cost of a modified solar still with rotating cylinder at $6~60/m3 with variation of

capital cost, interest rate, service lifespan and productivity [28]. Ayoub et al. stressed that environmental damage costs

should be considered to evaluate the economic feasibility of desalination technologies [28]. By combining the CO2

trading cost, external environmental degradation costs due to air pollution with the water costs from literature, water

costs of conventional fuel-based desalination technologies was estimated at $4.7-5.7/m3, which lies in the lower range

of renewable energy based desalination techniques.

Considering the low productivity, solar still is only recommended for small scale commercial application with the

capacity of less than 10 m3/d to supply freshwater for fisherman, small islands and small villages in remote areas.

Except for the efforts in performance improvement, to keep the simplicity, low maintenance and low cost feature of

solar still is also of vital importance. Thus, active solar still with extra solar thermal collectors should not be a future

focus. As pilot or real plants has not been reported in recent years, real application studies should be conducted to

further demonstrate and evaluate the applicability and economic feasibility of this technology in the present world.

6

Fig. 2 Approaches to achieving higher productivity in solar stills

Enhance

evaporation

Enhance

condensation Recover

Latent Heat

Capture and

converse more

solar energy

Better

evaporation

condition

Proper heat

storage

Increase

free surface

area of

water

Examples

- Place sponge

cubes in basin

water

- Stepped solar still

- Installing a

rotating cylinder.

Maintain

less water

depth

Internal

/external

condenser Cover cooling

Multi-effect

solar still

Examples

- Use phase change

materials (PCM), water

storage tank, sand under

basin liner, other materials

such as black rubber, black

gravel, etc.

Examples

- Usage of

sprinkler/ film

cooling on the

glass cover

Examples

- Double-basin still

- Double-effect

wick still

- Multi effect

diffusion still

Coupled

with

solar

thermal

collector

s

More

incident

solar

radiation

Increase

absorber

plate area

Better

absorber

material

Examples

- Coupled with reflector

- Install sun tracking device

- Optimization of glass

cover inclination

- Applying different cover

geometry design (such as

double slope or pyramid-shaped cover).

Example

- Integrate

fins in the

basin.

Examples

- Wick type solar still

- Stepped solar still

- Use floating

perforated black

plate or baffle

suspended plate.

Examples

- Apply

asphalt

basin liner

- Use black

ink or black

dye as

absorbing

materials.

Solar still modifications

7

Table 1 Summary of selected solar still studies

Authors year location Plant Type

System description Totally

passive or not

Feed water

Solar radiation

W/m2

Capacity 10-3 m3/d

Productivity L/m2.d

Productivity increase rate

(%)

GORso

SSEC

kWh/m3

Water

Cost US $/m3

Hansen et al.

[29] 2015

Tamil Nadu, India

9°11’N

Lab scale

Stepped solar still with water coral fleece and wire mesh on the absorber

plate. The absorber plate area was 0.75m2.

Passive - 488

e

(6am ~6pm)

4.3 5.71 71.2 0.64 k

1024 -

El-Samadony et al.

[30]

2015 Kafrelsheikh, Egypt 31°07’N

Lab scale

Stepped solar still with external condenser and air-suction fan. Basin

area was 0.5m2.

Vacuum fan used

Saline Water

578 e

(9am ~7pm)

2.7 5.5 66.6 0.54 1212 -

Stepped solar still with both internal, external reflectors and additional

external condenser and air-suction fan. Basin area was 0.5m2.

Vacuum fan used

- 4.5 9.0 165 0.66 993 36

El-Naggar et al. [31]

2015 Tanta, Egypt

30°47’N

Lab scale

Finned-basin liner still (FBLS)-single basin solar still integrated with

metallic fins. Basin area was 1m2. Passive

Brackish Water

590 e

(7am

~7pm）

4.8 4.80 11.8 0.55 1191 -

Kabeel et

al.

[26] 2014

Kafrelsheikh, Egypt 31°07’N

Lab scale

Single basin still with vacuum fan and external condenser.

Basin area was 0.5m2. Vacuum fan used (powered

by PV panel)

Saline Water

680 e

(7am ~5pm)

3.0 6.0 46.6 0.71 k

923 41

Single basin still with vacuum fan. Basin area was 0.5m2. Suspended

aluminum-oxide nanosized particles were mixed with feed water.

695 e

(7am ~5pm)

4.2 8.4 116 0.94 k

697

50

Same as above but without operating vacuum fan

Passive - 7.9 15.8 76 - -

El-Agouz et al.

[32] 2014

Tanta, Egypt

30°47’N

Lab scale

Stepped solar still with storage tank and continuous water circulation;

basin made of 10 steps with an effective absorber area 1m2 covered

with black paint.

Pump needed

Seawater

644 e

(10am ~7pm)

5.3 5.30 43 0.60 k

1092

35

Same as above while basin is covered with a layer of cotton cloth.

608 e

(10am ~7pm)

5.3 5.30 53 0.63 k

1039

Ayoub et al. [28]

2014 Lebanon Lab scale

Double-slope solar still with rotating cylinder. Basin area was 1 m2. The rotating cylinder was powered by a

photovoltaic (PV) panel

Rotating motor

powered by PV panel

seawater - 4.2 4.2 190 - - 6~60

8

Authors year location Plant Type

System description Totally

passive or not

Feed water

Solar radiation

W/m2

Capacity 10-3 m3/d

Productivity L/m2.d

Productivity increase rate

(%)

GORso

SSEC kWh/m3

Water Cost

US $/m3

Ahsan et al. [24]

2014

Selangor, Malaysia

Lab scale

Triangular solar still (TrSS) made from lightweight local materials with

a footprint of 0.8 m2. Passive

Saline water

- 1.3 1.60 - - - -

Rajaseenivasan et al. [33]

2014

Tamil Nadu, India

9°11’N

Lab scale

Double slope double basin solar still with black cotton cloth. Basin area

was 0.63m2 Passive

Saline water

497 (6 am

~6 pm) 3.5 5.57

-

0.60 1091 -

Omara et al. [34]

2013 Kafrelsheikh, Egypt 31°07’N

Lab Scale

Stepped solar still with internal reflectors installed on the vertical

sides of the steps. Absorber area was 1.16 m2

Passive Saline water

660 (9 am ~7pm)

7.4 6.35 75 0.56 1170 -

Ahsan

et al. [27]

2012 Japan / UAE

Lab scale

A tubular solar still (TSS) with a tubular cover and rectangular trough.

Footprint was 0.07 m2. Passive

Saline water

- 0.35 5.0 - - - 9.56

Tabrizi et al. [35]

2010 Zahedan,

Iran 29°29’N

Lab scale

Weir type cascade flow stepped solar still with PCM (paraffin wax under the absorber plate). Absorber area

was 0.45 m2

Passive - 610 j

2.2 4.85 - 0.53 j,k

1235 -

El-Sebaii et al.

[36] 2009

Jeddah, Saudi Arabia

21°42’N

Mathematica

l model

Single slope, single basin solar still integrated with 3.3cm of PCM

(stearic acid) beneath the basin liner; basin area 1m2

Passive - 542

e

(7 am ~7pm)

9.0 9.0 44.5 0.85 771 -

Kumar et al.[37]

2009

New Delhi, India,

28°36’N

Lab scale

Single slope solar still coupled with PV/T system; basin area was 1m2.

The PV/T system included two 2 m2

flat plate collectors and a 0.66 m2 PV module integrated at the bottom of

one collector.

Integrated with flat

plate solar collectors and PV panel

ground water

- 4.6 g

4.63 g

250 - - 49~143

Kabeel et al. [38]

2009 Tanta, Egypt

30°47’N

Lab scale

Solar still with four side pyramid shape cover and concave wick basin.

Basin aperture area was 1.44 m2. Passive

Saline water

731 e

(10 am ~7pm)

4.0 2.78 - 0.28 k

2340 65

Abdallah

et al. [39] 2008

Amman, Jordan,

31°56’N

Lab scale

Single slope still integrated sun tracking system, basin area 1m2.

With sun tracking system

Saline water

600 e

(7am ~6pm)

- - 22 - - -

Abdel-Rehim et

al. [40] 2007

Cairo, Egypt

30°03’N

Lab scale

Single solar still coupled with parabolic trough collector, copper

pipe serpentine loop heat exchanger installed in the bottom of the still to

transfer heat.

Coupled with

parabolic trough

collector

Saline water

635 e

(9am ~7pm)

2.6 - 18 - - 12

9

Note: a. GORso (gain-output ratio-solar) in this table is defined as the energy ratio of total latent heat of distillate to the total input solar energy to solar still. The subscript ‘so’ was used to make a difference between the GOR definitions in other thermal desalination technologies. Normally this parameter is referred to as thermal efficiency (�) in solar still studies. It can be determined by the equation below.

�����(�) =��·∆��

�.���·� (1)

��, average distilled water mass flow rate, kg/h;

∆��, latent heat for evaporation, kJ/kg;

Aa, total area of absorber; m2

G, solar radiation intensity over absorber area, W/m2.

b. SSEC (specific solar energy consumption) refers to the amount of input solar energy consumed per m3 fresh water production. The relationship between SSEC and GORso is:

SSEC,� !

"#=

∆��

�.��$%&' (2)

∆�� at 60℃ (2358 kJ/kg) was used in the calculation of GORso and SSEC when the values are not available in literature. All SSEC values in this table were calculated with equation (2) by the authors.

c. Productivity in this table refers to the amount of water produced per m2 basin area per day. Productivity increase rate refers to the productivity increase in comparison with the single basin still before the specific modification in literature.

d. As the performance of solar still will be largely influenced by meteorological conditions, the solar radiation, productivity, capacity, GORso and SSEC in this table mostly refer to the parameters for selected testing days. The exceptions will be specified below.

e. These are the average solar radiation data in the testing days between certain periods of time during which data was available in the solar radiation curves.

f. This is converted from the typical daily solar radiation in July of Aswan, Egypt by assuming 10 hours sunshine per day.

g. These values are average yield and productivity of 260 clear days in a year.

h. This is the summer time capacity of the plant.

i. This is the designed capacity of the plant.

j. The average solar radiation of testing day was obtained from local meteorological station. 10 hours sunshine was assumed in the calculation of GORso.

k. These GORso values were calculated with equation (1) by the authors.

m. Some of the water cost values were converted from local currency to US dollars with the exchange rate at the paper received date available in internet by the authors.

Authors year location Plant Type

System description Totally

passive or not

Feed water

Solar radiation

W/m2

Capacity 10-3 m3/d

Productivity L/m2.d

Productivity increase rate

(%)

GORso

SSEC kWh/m3

Water Cost

US $/m3

Fath et al. [41]

2003 Aswan, Egypt,

24°05’N

Mathmatica

l model

Pyramid shaped single basin still, basin area 1.53 m2

Passive - 694 f

6.5 4.25 - 0.4 k

1638 30

Delyannis

et al. [42] 1995

Kimolos island, Greek,

36°48’N

Real plant

Absorber area 2008 m2 Passive Seawater - 12500 h

5.97 - - - -

Tiwari et

al. [43] 1984

New Delhi, India,

28°36’N

Real plant

The plant consisted of 28 multi-wick solar stills (1 m2 basin area each),

1000L large storage tank and some small storage tanks and pipes.

Passive - - 70 i

2.5 - - - -

10

2.2 Indirect solar desalination

2.2.1 Solar humidification-dehumidification

Humidification-Dehumidification (HDH) technology utilized the moisture carrying capacity of hot air

to realize the separation of saline water and pure water (moisture). Unlike solar still, which is mostly

passive, HDH is normally coupled with external heaters such as solar collectors. Also, humidification

and dehumidification take place in separate components, which allows each individual element to be

designed and optimized independently [44]. Consequently, much higher thermal efficiency could be

achieved in HDH than solar still. The major components of HDH process are humidifier, dehumidifier

and external heater [45]. Solar HDH desalination is recognized as a suitable choice for decentralized

water production with relatively low capacity.

Fig. 3 Schematic diagram of solar HDH configurations: (a) water or air heated closed air, open water

cycle (CAOW); (b) water or air heated closed water, open air cycle (CWOA); (c) water or air heated

open water, open air cycle (OWOA).

11

HDH process can be classified into three categories based on the cycle configuration: closed air, open

water cycle (CAOW), closed water, open air cycle (CWOA), and open water, open air cycle (OWOA).

The configurations are shown in Fig.3 while descriptions can be found in Sharon et al. [10] and Wael

et al [46]. Either air or the feed water will be directly heated with external energy source in the HDH

cycle (AH and WH are used as abbreviations for air heated system and water heated system in the text

below). Solar thermal collectors for both water and air heating can be applied in solar HDH process.

Most of the investigations on solar HDH are concerned about the improvement on productivity and

efficiency of the system, which can be made by design and optimization of the HDH cycle and

individual element, as well as the optimization of operating conditions, such as the air/water mass

flow rate and temperature of feed water and inlet air [47].

Table 2 shows a list of selected solar HDH studies reported in recent years. Al-Sulaimain et al. [44]

studied the performance of an OWOA HDH system integrated with the parabolic solar air collector

based on thermodynamic model while comparing two different configurations in which the solar

collector was placed at different position in the HDH cycle, i.e. before the humidifier (first

configuration) or between humidifier and dehumidifier (second configuration). The second

configuration showed significantly higher average GORso (defined in Table 2) value (4.7) than the

first configuration (1.5). As a result, much higher productivity was achieved in the second

configuration with annual average value of 954kg/d rather than 293kg/d for the first one. HDH

process can also be designed into multi-stage systems (sometimes referred to as multi-effect in

literature). A 500 L/d two-stage CAOW-WH solar HDH pilot system was designed and constructed in

Qom, Iran by Zamen et al. [48]. A mathematical model was developed for multi-stage solar HDH

process as well. The multi-stage solar CAOW-WH HDH described in this study was featured by

several smaller closed-air loops between humidifier and dehumidifier with each small loop stand for

one stage. Modelling results showed that the specific water production was significantly improved by

more than 40% in a two-stage HDH process, while only 4% and 1% increment can be achieved than

lower-stage process for 3- and 4-stage HDH processes, respectively. Considering the increased

desalination unit cost with increased stages, two-stage HDH process was regarded as the most suitable

choice. The pilot system was tested during summer and winter days. Specific water production could

reach 7.25 L/ m2 �d in summer days, which was about 40% higher than single-stage unit tested in

previous study.

Different types of humidifiers and dehumidifiers utilized in HDH process have been extensively

reviewed by Narayan et al [49]. As shown in Table 2, packed bed humidifiers and finned-tube heat

exchanger (dehumidifiers) are mostly adopted in recent solar HDH applications.

12

Solar thermal collectors such as flat plate solar collector, evacuated tube solar collector, parabolic

trough solar collector have been adopted in solar HDH studies. However, compared with widely

commercialized solar water heating devices, there is less experience in terms of solar collectors for air

heating and the technology is relatively immature. Standard thermal efficiency of a few commercial

solar air heater were reported as only 10.2%-32.3% [49]. Developing high efficient solar air collector

specially for solar HDH applications have received considerable attention [50-53]. Summers et al. [51]

designed a novel flat-plate solar collector for air heating with built-in phase change material (PCM)

paraffin wax for HDH desalination. It was found that the solar collector with 8cm PCM layer below

the absorber plate could produce constant output temperature with an average collector thermal

efficiency of 35%. In the solar HDH pilot system developed by Li et al. [50], a new evacuated tube

solar air collector was designed and adopted. It contained 20 dual-wall glass tubes connected in

parallel and mounted on a insulated metal frame equipped with a header for air supply and return. The

thermal efficiency of the solar air collector could reach 47% with optimized air flow rate (140m3/h).

Reported GORso of HDH varies between 0.92~4.7, significantly higher than that of solar still. The

corresponding specific solar energy consumption (SSEC) was calculated as 139~712 kWh/m3. The

specific water production was reported in the range of 0.36~19.08 L/ m2 �d. The two lowest values

reported by Soufari et al. [54] and Zhani et al. [55] were all lab scale studies with very small capacity

(10L and 20L) in which the solar collector may have been oversized compared to the actual

requirement of the HDH system. Water cost was estimated in a range of $2.90~$500/m3 with the

higher cost range $115~500 /m3estimated by Dayem [56, 57] and Zhani et al. [55] based on lab scale

systems with 10~20L/d daily production while the lower cost range was $2.90~22.1 /m3 for the data

from pilot scale studies. According to Narayan et al. [49], for small capacity desalination plants, the

water cost can go up to $3.00 /m3 for the currently most economical RO desalination system. It seems

that solar HDH could be economically comparable with conventional desalination plants in small

scale applications. It has also been suggested by Li et al. [13] that HDH system should be targeting at

small scale desalination plants with 5~100 m3/d capacity.

Presently, solar HDH process is still under research and development. Very limited work has been

done in identifying the advantages and disadvantages of different configurations, the influence of

which is considered considerable on HDH performance [47]. In order to commercialize this

technology, further optimization of HDH cycle and the three major components will remains to be the

most important focus of HDH research. Besides, better understanding of the thermodynamics of solar

HDH process and development of mathematical model are also crucial for the scaling up of the

process.

13

Table 2 Summary of selected solar HDH Plants

Authors year location Plant type

Energy Source System description Feed Water

Solar radiation

W/m2

Capacity (m3/d)

SWP L/ m2 �d

GORso

SSEC Specific solar

energy consumption

kWh/m3

Water

cost ,m

US $/m3

Al-Sulaiman

et al.

[44]e

2015

Dhahran, Saudi Arabia

26°16’N

Theoretical modelling

Parabolic trough solar air collector

(PTC)

OWOA-AH system, PTC air heater located before the humidifier. The length of parabolic trough was 10m, the width was 5m. -

150~750

0.293 5.86 1.5 437 -

Same as above except that PTC was located between humidifier and dehumidifier.

0.954

19.08 4.7 139 -

Zamen et

al. [48]

2014 Qom, Iran 34°38’N

Pilot plant

Flat-plate solar collector

(FPC)

Two stage CAOW-WH system, polypropylene packed bed humidifier; finned tube condenser used in dehumidifier; 80m2 FPC heated water indirectly via heat exchanger.

Saline water

- 0.58f

7.25f - - -

Kabeel et

al. [58] 2014

Tanta, Egypt

30°47’N

Lab scale

Flat plate solar water heater and solar air

heater

Hybrid solar desalination system consisted of a CWOA HDH unit, a single stage flash evaporation unit and solar water/air heating system; packed bed humidifier; counter-flow shell and multi-pass tube heat exchanger used as dehumidifier. Nano-fluid was used as heat transfer fluid for the solar water heater loop.

Seawater 670 g

(9am-6pm) 0.042

h - - - -

Chang et

al. [59] 2014 China

Pilot plant

Thermo- -syphon water

heater

Two stage CAOW-WH HDH system, two successive humidifiers and dehumidifiers, operating at higher and lower temperature, respectively; packed bed humidifier filled with porous plastic balls and dehumidifier with heat exchanger made of corrugated aluminum finned copper tubes were adopted.

Seawater - 0.5 i

- - - 4.4

14

Authors year location Plant type

Energy Source System description Feed Water

Solar radiation

W/m2

Capacity (m3/d)

SWP L/ m2 �d

GORso

SSEC Specific solar

energy consumption

kWh/m3

Water cost US $/m3

Dayem [56]

2014

Makka, Saudi Arabia

21°25’N

Lab scale

Flat plate solar collector

CAOW-WH HDH unit with one single dual wall storage and desalination tank. The inner space functioned as humidifier while outer layer acted as the condenser. 1.15 m2 flat plate solar collector was used.

Saline water

- 0.010 h

9 h

- - 500

Yuan et

al. [60] 2011

Beijing, China

39°55’N

Pilot plant

Evacuated tube solar air

heater and solar water

heater

CWOA air/water heated HDH system; honeycomb-structured pad humidifier and dehumidifier with fin-tube condenser; 100m2 solar air heater,14m2 solar water collector

Seawater 500 k

1.0 j

8.77 2.0 328 2.9

Dayem

[57] 2011

Cairo, Egypt

30°03’N

Lab scale

Flat plate solar collector

CWOA-WH HDH system with a 100L insulated galvanized tank as desalination unit. An built-in air atomizer to eject the hot water used for humidification and 2 containers condenser used for dehumidification. 2.35 m2 flat plate solar collector was

used as heat source.

Saline water

- 0.010 h

4.25 h

- - 200

Zhani et

al. [55] 2010

Sfax, Tunisia 34°44’N

Lab scale

Flat plate air/water solar

collector

CAOW air/water heated HDH system with a pad humidifier, an evaporator, and a dehumidifier; 16m2 air solar collector and 12m2 water solar collector

Seawater 689 g

0.021 h

0.71 h

- - 115.2

Mathioulakis et al.

[61] 2010

Geroskipou, Cyprus 34°46’N

Pilot plant

Flat plate solar collector

CAOW-WH HDH system, 1000L/d commercial HDH unit, flat plate solar collector with surface area 96m2 and a 5 m3 thermal storage tank.

Saline water

- 1.0 j

10.41 j

- - 22.1

Soufari et

al. [54] 2009

Karaj, Iran

35°50’N

Lab scale

Flat plate solar collector

CAOW-WH HDH system, packed bed humidifier, fin-tube heat exchanger used as dehumidifier; 28 m2 flat plate solar collector.

Saline water

- 0.010 j

0.36

j

- - -

Yamali et

al. [62] 2008

Ankara, Turkey

39°56’N

Lab scale

Double pass flat plate solar air collector

CWOA-AH HDH; pad humidifier with four plastic pads mounted vertically; heat exchangers made with copper tubes and corrugated aluminum fins used as dehumidifier; 0.5 m2double-pass flat-plate solar collector.

Saline water

814 g

(10am ~4pm)

0.004 h

8.0 h

0.92

l

712 -

15

Authors year location Plant type

Energy Source System description Feed Water

Solar radiation

W/m2

Capacity (m3/d)

SWP L/ m2 �d

GORso

SSEC Specific solar

energy consumption

kWh/m3

Water cost US $/m3

Houcine et al. [63]

2006 Kairouan, Tunisia 35°40’N

Pilot plant

Four-fold-web-plate solar air collector

CAOW-AH HDH with successive 4 stage air heating and humidification process prior to the dehumidifier. Pad humidifier, finned-tube heat exchanger dehumidifier was adopted. Total surface area of solar air collector was 205m2.

Seawater - 0.564 h

2.75h - -

-

Note:

a. GORso (gain-output ratio-solar) in this table is defined as the energy ratio of total latent heat of distillate to the total input solar energy to the HDH system. It can be determined by the equation (1). Aa in the equation refers to total aperture area of solar collectors instead of absorber area.

b. SSEC (specific solar energy consumption) refers to the amount of solar energy (that reached the aperture area of solar thermal collectors) consumed per m3 fresh water production. The relationship between SSEC and GORso is shown by equation (2). ∆�� at 60℃ (2358 kJ/kg) was used in the calculation of GORso and SSEC when the values are not available in literature.

c. SWP (specific water production) refers to the water production per m2 of solar collector area per day [46].

d. The solar radiation, GORso, SSEC values in this table mostly refer to the parameters for specific testing days or average value of testing days; capacity and specific water production (SWP) in the table refers to the fresh water production in specific days or the designed capacity for the pilot or real plant. The descriptions of these values will also be specified below.

e. The solar radiation in this reference refers to the range of monthly average values throughout a year while GORso, capacity and SWP refers to the annual average values.

f. The capacity and SWP was derived from daily production rate on typical summer days in this reference.

g. These are the average solar radiation data in the testing days between certain periods of time during which data was available in the solar radiation curves.

h. These values are based on experimental production rate in specific testing days.

i. This is estimated daily production rate based on the steady state performance of the unit.

j These values are based on designed capacity of the pilot plants or experimental setups.

k. This is the daily average solar radiation when the daily production met the designed capacity.

l. These GORso values were calculated with equation (1) by the authors.

m. Some of the water cost values were converted from local currency to US dollars with the exchange rate at the paper received date available in internet by the

authors.

16

2.2.2 Solar-powered MSF

Multi-stage flash (MSF) is currently the most dominant thermal desalination technology, which has

around 21% share of worldwide desalination installation capacity, being second only to reverse

osmosis [64]. MSF evaporator consists of several consecutive stages with decreasing pressures. As

shown in Fig. 4, the feed seawater/brackish water is first preheated by the condensation of vapour

while flowing through these stages in tubes. Then the preheated brine receives external heat from a

brine heater normally with heating steam, after which it successively passes through stage by stage,

where sudden boiling (flash) of the feedwater takes places as a result of the reduced pressure.

Fig. 4 Schematic diagram of a solar MSF desalination system with thermal storage

In solar powered MSF, solar thermal collectors are used in connection with a conventional MSF

desalination system. The selection of solar thermal collector, proper design of solar heating cycle,

design and optimization of MSF unit are all of vital importance to the successful application of solar

powered MSF process. The performance and GOR (gain output ratio) of MSF plants could be

improved by increasing TBT (top brine temperature, temperature of the brine entering the first

flashing stage), reducing intake saline water temperature , increasing numbers of stages and specific

heat transfer area. Since relatively high and stable TBT (normally 90~120℃ in a conventional MSF

plant) is required, an effective thermal storage system used for thermal buffering is favorable in solar

MSF systems.

17

Some selected solar MSF studies are listed in Table 3. Several solar MSF pilot plants have been

reported since 1980s with the capacity of 10~20 m3/d [65]. A 10 m3/d self regulating solar MSF pilot

system was designed and tested in Safat, Kuwait in 1983 [66]. A 7 m3 thermal storage tank was

installed in the system which served as a thermal damper between the solar collector and the MSF

subsystem. The reported specific thermal energy consumption (STEC) of the MSF subsystem was in

the range of 83–105 kWh/m3 with the corresponding GOR being 6.5–8.

Fig.5 Schematic diagrams of different solar MSF-BR desalination plants proposed by Mohamed et

al. [67]

Very limited publications on solar MSF plants are available in the past 20 years. Meanwhile, reported

solar MSF studies are mostly focused on low capacity plants. Only recently, Eldean et al. [67]

analyzed and evaluated a 5000 m3/d solar MSF-BR (multi-stage flash brine recycle) system with

18

REDS-SDS software package in Matlab/Simulink environment. Three different configurations

(shown in Table 3 & Figure 5) of the solar thermal cycles were proposed to be integrated with the

MSF unit: 1) direct vapor generation (DVG) , 2) indirect vapor generation (IDVG) , 3) The

combination of MSF unit with solar organic Rankine cycle (SORC) for electricity-water cogeneration

(Description see Table 3). Design parameters such as solar field dimensions, heat exchanger details,

flow rates, operating conditions, etc. were derived from the software with basic input data including

capacity, weather conditions, top brine, seawater and blow down temperatures, mechanical

efficiencies of turbo machinery units, etc. A GOR of 12 were achieved with 40 stages MSF. The

water cost of the proposed plant were estimated at $1.36~1.58/m3, which could be comparable with

conventional MSF plants powered by fossil fuels. This study indicates that solar MSF can be an

economically and technically viable option for large scale desalination plants.

Compared to its alternative technology solar MED (multi effect distillation, see section 2.2.3)，reported research and applications are far less in solar MSF. This indicates that solar MSF may be

either technologically or economically less competitive than solar MED probably due to three major

reasons: 1) the high TBT requirement makes it relatively less favorable to be combined with solar

energy; 2) with higher TBT, it will exhibite higher fouling and scaling trend; 3) MSF is

thermodynamically less efficient than MED. However, as the present mostly installed thermal

desalination technology, the transformation of conventional fossil fuel based plants into solar-fuel

integrated plants may be a future need with the depleting of fossil fuel resources. Thus further

research should be conducted in solar MSF to realize high efficiency integrated systems. Besides, dual

purpose plants that combines concentrating solar power plant with solar thermal desalination is also a

potential option for future MSF development. Further information of CSP+D plants has been

presented in section 2.2.3.

19

Table 3 Summary of selected solar powered MSF studies

Authors year location Plant type

Energy Source System description Feed Water

Solar radiation

W/m2

Capacity (m3/d)

TBT ℃

GOR

STEC kWh/m3

GORso Water cost US $/m3

Sharaf Eldean et

al. [67] 2013 Egypt

Modeld

Parabolic trough collector (PTC)

DVG Solar MSF- BR system consisted of PTC solar field, brine heater, 40 stages MSF-BR unit. Brine was heated by the steam which is directly generated from solar collector. Water was used as working fluid for the solar thermal cycle. Collector outlet steam temperature 135℃. Solar collector area 61680m2

Seawater 594

e 5000 90~130

12 53

g -

1.36 IDVG solar MSF-BR system, similar as above but HTO (heat transfer oil, Therminol-VP1) was used as working fluid in solar field, HTO transferred energy to steam via a boiler heat exchanger (BHX) unit. Seawater was heated up by steam via brine heater. Collector outlet temperature 350 ℃ , brine heater steam temperature 135℃. Solar collector area 61680m2.

MSF combined with solar organic Rankine cycle (SORC), the system was consisted of 40 stages MSF-BR unit, pumps, PTC solar field, BHX unit, turbine expander unit and condenser unit. The condenser in ORC loop was used as brine heater for the MSF unit. HTO and toluene was used through the solar field and the ORC, respectively. Collector outlet temperature 350℃, brine heater steam temperature 135℃. Solar collector area 93050m2.

1.58

Nafey et al.

[68] 2007

Suez, Egypt

29°58’N

Lab scale

Flat plate collector (FPC)

The system consisted of a flash chamber, a condenser unit with copper tube heat exchanger, and a flat plate solar collector with surface area of 2.39m2.

556 f

(9am~4pm) 0.011

f 49~56 - - 0.7~0.9 -

20

Authors year location Plant type

Energy Source System description Feed Water

Solar radiation

W/m2

Capacity (m3/d)

TBT

℃

GOR STEC

kWh/m3 GORso

Water cost US $/m3

Lu et al.

[69] 2001

El Paso, US

31°47’N pilot Solar pond

A small 3-effect, 4-stage flash distillation unit powered by solar pond. LCZ (lower convective zone) brine from a 3000m2 pilot solar pond with temperature of 77~87℃ was used to heat up the feed water.

Saline water

- 2.3~7.2 63~80 1.7~3.7 h

175~380

- -

Moustafa et

al. [66] 1985

Safat, Kuwait

29°22’N Pilot PTC

A self-regulating 12 stages MSF system, energy derived from 220 m2 PTC solar field. A 7m3 thermal storage tank was installed between them. Heat exchanger was used as interface between MSF subsystem and the thermal storage tank.

Seawater

- 10 40~90 6.5~8 83~105 - -

Note:

a. GOR (gain-output ratio) in this table is defined as the energy ratio of total latent heat of distillate to the directly input thermal energy to drive MSF process (sometimes referred to as performance ratio in literature). Normally it is the latent heat of heating steam that enters brine heater. At this case, GOR is equal to the ratio of distillate mass flow rate to the heating steam mass flow rate. b. GORso (gain-output ratio-solar) in this table is defined as the energy ratio of total latent heat of distillate to the total input solar energy to the MSF system. It can be determined by the equation (1). Aa in the equation refers to total aperture area of solar collectors.

c. STEC (specific thermal energy consumption) refers to the amount of input thermal energy consumed per m3 fresh water production. STEC can be calculated with GOR value when these two factors refer to the same thermal desalination process. In that case, the relationship between STEC and GOR is:

STEC,� !

"#=∆��,*

�$% (3)

Where ∆��,+ refers to the latent heat of evaporation at a reference temperature (assumed as the evaporation temperature of produced steam).

d. The simulation was based on a visual modular program (REDS-SDS software package) developed by [70] using Matlab/Simulink environment.

e. This is the daily average value of the input deigned solar radiation. 10h daylight hour was assumed in the calculation.

f. This is the average solar radiation between 9am~4pm of the testing day. The capacity is the daily production of the same day.

g. The STEC value was calculated with the available GOR value, ∆��,+ at 90℃ (2383 kJ/kg) was used in the calculation.

h. The STEC value was calculated with the available GOR value, ∆��,+ at 70℃ (2336 kJ/kg) was used in the calculation.

21

2.2.3 Solar-powered MED

Fig. 6 Schematic diagram of solar MED desalination system with feed preheating

In multi effect distillation (MED) (see Fig. 6), seawater/brackish water is delivered to several cells (i.e.

‘effects’) maintained at low pressure successively. External energy is supplied to the first effect.

Latent heat of vapourization is recovered as the succeeding effect serves as the condenser for the

vapour produced in the previous effect. While MSF is currently the predominant thermal desalination

technology adopted in large scale plants, MED is regraded as thermodynamically more efficient and

could be operated at much lower top brine temperature (TBT) (55~120℃) to avoid scaling and

corrosion problem [71, 72]. MED process has received considerable attention regarding solar powered

systems due to the low TBT requirement.

(a) (b)

22

(c)

(d)

Fig.7 Schematic diagram of four heat pumps used in MED (a) Thermal vapour compression (TVC),

(b) Mechanical vapour compression (MVC), (c) Absorption heat pumps (AHP), (d) Adsorption heat

pumps (ADHP)

The performance of MED plants can be significantly affected by design and operating parameters

such as number of stages, top steam temperature (heating steam temperature of the first effect),

heating steam flow rate, temperature difference in the final condenser, etc. Except for optimizing

these parameters, four types of heat pump have also been adopted for coupling with MED (Fig. 7) to

achieve higher efficiency in MED system, including mechanical vapour compression (MVC), thermal

vapour compression (TVC), absorption heat pumps (AHP) and adsorption heat pumps (ADHP) [13].

To some extent, these heat pumps all aims to recover the last effect steam or the low grade energy of

it. Among them, TVC have been widely used in commercial desalination plants since 1990s [72].

TVC features the steam-jet ejector based on Venturi principle through which the pressure and

temperature of the steam generated in the last effect of the MED process is elevated by introducing

high pressure motive steam. As the last effect steam is partly reused, the required motive steam, the

size of thermal energy supply system and the final condenser are drastically reduced. In MED-MVC,

the low temperature/pressure steam from the last effect of MED is compressed to high

temperature/pressure steam by mechanical compressor which is mostly driven by elctric power or

diesel engine. Final condenser is eliminated in this configuration which make it even more compact.

However, presently MVC application is limited to small to medium desalination plants with maxmum

5000 m3/d capacity [73]. Hygroscopic absorbent/adsorbent is utilized in AHP and ADHP, respectively.

Four basic heat exchange units are included in a typical single-effect AHP loop: evaporator, absorber,

generator and condenser. When combined with MED [74], evaporation of refrigerant (water) is

induced by recovering latent heat from the MED last effect steam. External thermal energy is

provided to the generator so that the high temperature steam is produced which serves as the heating

23

steam of the first MED effect. In a MED-ADHP hybrid system, vapour produced in the last effect of

MED is adsorbed in the adsorber/desorber bed. Silica gel and zeoliote are the most commonly used

adsorbents. This combination enables the last effect to be operated below ambient temperature, and

thus more effects can be inserted at the same TBT . Since adsorption and desorption cannot take place

simultaneously, the AD cycles are batch type. Two or multi adsorber bed configuration is essential for

continuous production. MED-ADHP is regarded as presently infeasible for commercial desalination

apllications due to poor performance and the operating challenge brought by its batch feature [75].

Table 4 lists selected solar powered MED systems reported in recent years. Solar desalination based

on MED have been investigated and tested in the Spanish solar research centre Plataforma Solar de

Almeria (PSA) since 1988 with the implementation of STD project (1987-1994) followed by

AQUASOL project (2002-2006). Several types of solar MED systems have been tested and evaluated

[74, 76-78], including the MED powered by parabolic trough solar collectors (PTC), MED-TVC

system driven by high pressure steam derived from a small solar thermal power plant, and MED-

DEAHP system driven by PTC or hybrid solar-gas energy source. A 14-effect forward-feed

commercial MED unit with nominal distillate production rate of 3 m3/h was adopted in these systems.

Two prototypes of DEAHP (double effect absorption heat pump) system have been designed and

tested with the first one drove the MED first effect by directly supplying low-pressure steam whereas

the second one drove the first effect by providing hot water via auxiliary hot water tanks. Several

advantages of connecting AHP with MED were pointed out by Alarcon-Padilla et al [74], including

reduction of thermal energy consumption, which result in significant reduction of solar field size in

the case of solar desalination; decrease of electrical energy consumption and seawater intake capital

cost because of the reduced cooling seawater flow requirement; possibility of reducing last effect

operating temperature below ambient allowing extra effects at same TBT.

Solar powered MED-AHP was also adopted in a recent study by Stuber et al [75]. The pilot plant was

implemented and operated in California, USA which aims to reuse subsurface agriculture drainage

water after desalination. The open-loop AHP system mainly consisted of an absorber and a steam

generator (desorber) while the MED unit worked as evaporator and condenser of a traditional AHP

circuit. Low temperature steam from last effect of MED was fed to the absorber. High temperature

steam was produced in the generator via heat exchange with the heat transfer fluid from 656 m2

parabolic trough collector solar field, after which it condensed in the first effect of the MED system.

By comparing two different operation mode (MED only and AHP-MED operation), it was found that

specific thermal energy consumption (STEC) was largely reduced (by 49%) with the heat pump, a

minimum STEC of 133.2kWh/m3 compared to 261.87 kWh/m3 without heat pump. This result also

indicated a 49% reduction in solar collector area requirement at the same freshwater production rate.

Based on the pilot system, an optimized commercial system using a 10-effect MED and an open-

24

cycle double-effect AHP was simulated. The STEC of the system was estimated at 34.9 kWh/m3 with

the GOR of MED as 18.4.

The integration of concentrating solar power plants (CSP) and desalination technologies (CSP+D) has

also attracted some research interests in recent years. With the merit of cogeneration of electricity and

fresh water, CSP+D could become a sustainable solution to solve both power and water problems

while reducing the solar power and desalination water cost when compared with independent plants

[79]. The CSP+D concept is especially suitable for middle or large scale solar desalination

applications. Low temperature MED (LT-MED) and Reverse Osmosis (RO) are regarded as the most

promising desalination technologies to be coupled with CSP [79]. Although no real plant has been

built yet, CSP+MED have been investigated by researchers based on simulation and different

configurations have been discussed [79-82]. The size of CSP could be designed either to fit the MED

desalination plant when all the exhaust steam from the turbine are used to drive MED or with larger

size just to meet the electricity capacity required. A techno-economic analysis of CSP+MED plant

configurations for two sites in Isral and Jordan was conducted by Olwig et al [81]. Desalination plants

with 24,000 m3/d capacity were designed and simulated. The CSP plant capacity (42 MW electricity

generation capacity) was selected to meet the exhaust steam requirement of MED plant. Thus MED

unit replaced the cooling subsystem of the power generation cycle. Simulation result showed that the

water cost in theses two plants were $0.943/m3 and $1.215/m3, respectively. This lies in the water cost

range of fossil fuel powered large scale commercial MED plants reported in literature [83], indicating

that CSP+MED is an economically realistic option for fresh water supply in the MENA region

(middle east and north africa). It should be noted that ‘power credit method’ was applied in the

calculation of unit water price, in which the steam cost for MED plant was estimated by assuming that

the power loss due to the use of backpressure turbine instead of condensing turbine adopted in

independent power plants has to be compensated by purchasing power from the grid. Casimiro et al

[80] simulated a CSP+MED system with a CSP+MED-TVC integrated plant model developed based

on the CSP simulation model-SAM physical trough model developed by NREL (U.S. National

Renewable Energy Laboratory). Considering the high intermittence of CSP plant, the MED plant was

downsized compared with the CSP installed capacity so that it can be operated more frequently under

designed conditions. Seawater cooling circuit was essential in this case as the heat load output from

the CSP plant is only partly utilized by the MED plant. A 36,112 m3/d MED desalination plant was

simulated at 40% heat load output from the 110 MWe gross capacity CSP plant. When simulated with

the weather condition in Sicily, Italy, 34.2% yearly installed capacity of the CSP plant and 41.4%

yearly capacity of the MED plant could be achieved.

As shown in Table 4, reported GOR values of the desalination unit in solar MED plants are in the

range of 2.7~15, with the lowest value correspond to a 3-effect forward feed MED system and the

highest value correspond to a 16-effect MED-MVC system. Water cost were estimated at $4.1~22/m3

25

for small to medium solar MED plants with the capacity between 2.3~160 m3/d; while in contrast

estimated water cost for large scale solar MED plants or CSP+MED plants are much lower, being in

the range of $0.94~1.32/m3. This is comparable with the conventional large scale thermal

desalination plants. Till now, significant progress has been made in the research and development of

solar MED systems. Solar MED could be a sustainable alternative for middle to large scale

conventional desalination plants though no large scale plants have been built yet. Dual purpose plants

that combines CSP has no real case as well. Further research based on demonstration and

thermodynamic modelling need to be done to realize the scaling-up of solar MED as well as to

demonstrate the technical and economical feasibility of this technology. Meanwhile, reducing the

environment impact of solar MED plants by effectively managing the brine should also be a future

focus.

26

Table 4 Summary of selected solar powered MED studies

Authors Year Location Plant Type

Energy source

System Description Feed

Water Capacity (m3/d)

GOR STEC

(kWh/m3) SEEC

(kWhe /m3) Water cost US $/m3

Stuber et

al. [75] 2015

Firebaugh, US 36°51’N

Pilot Plant

Parabolic trough

collector (PTC)

Solar-AHP-MED pilot system, 3-effect MED with plate and frame evaporator; an open-loop absorption heat pump integrated with MED;PTC solar thermal system with 656m2 aperture area, heat transfer fluid was circulated through solar array and delivered heat to the steam generator of AHP, a set temperature of 180℃ for heat transfer fluid was used in operation.

Brackish agricultur

al drainage

water

6.74d

5.27

133 - -

Casimiro et

al. [80] 2014

Sicily, Italy 37°30’N Model

g

Parabolic trough CSP (concentrati

ng solar power) plant

CSP+MED-TVC plant: CSP plant with design gross output of 110 MWe (design net output) at designed solar radiation 950W; 12-effect MED-TVC plant operates at 40% CSP heat load output (an extra condenser absorb the rejected heat load from CSP); MED feed saturated steam temperature 64.5℃. Yearly capacity factor of CSP and MED plant are 34.2% and 41.4%, respectively.

Seawater 36,112

10.2

64e 2.81 -

Liu et al.

[84] 2013

Dalian, China

38°55’N

Mathematical

Model

Evacuated tube

collector (ETC)

and auxiliary electrical

heater

Major component of the system included ETC subsystem, thermal storage tank, a flash evaporator (produced driving steam for MED), MED subsystem and auxiliary electrical heating/cooling devices. Solar collector area 4000m2; 10 effect MED with the first effect steam temperature 66℃.

Seawater 160 - - - 4.1g

Kim et al.

[85] 2013

Jeddah, Saudi Arabia

21°32’N

Numerical Model

ETC and auxiliary

heater

Major component of the system included a 6-effect backward feed MED, 849 m2 ETC solar collector, 280m3 hot water storage tank and auxiliary heater. When heating water temperature was set at 80℃, annual solar fraction was 49.4%.

Seawater 15.6 4.11 159e - -

27

Authors Year Location Plant Type

Energy source

System Description Feed

Water Capacity (m3/d)

GOR STEC

(kWh/m3) SEEC

(kWhe /m3) Water cost US $/m3

Olwig et

al. [81] 2012

Aqaba, Jordan 29°31’N

Modelh

Parabolic trough CSP

plant, natural gas as backup

Two 10-effect MED units with horiaontal tube falling film evaporators, coupled with an Andasol-type CSP plant (mainly consists of parabolic trough collectors, molten salt thermal storage tanks and Rankine steam power cycle). 42MWe power generation capacity was selected to meet the steam consumption of MED plant. Designed top brine temperature (TBT) 65℃.

Seawater 24,000

8.35 77.8 2.4 0.943

Ashdod, Israel

31°48’N 8.52 76.3 2.3 1.215

Sharaf et

al. [73] 2011 Egypt Model

j PTC

Solar MED-TVC system mainly consisted of PTC solar collector field, 5-effect parallel feed MED-TVC unit and the boiler heat exchanger (BHX) which generated motive steam that directly drive the TVC section. Total solar field area was 94760 m2. TBT in the first effect was 58.5℃

Seawater 4545

8.04 81.5e 1.58~2.0 1.32

Solar MED-MVC system mainly consisted of PTC solar collector field, organic Rankine cycle (ORC) for electricity production, the boiler heat exchanger (BHX) which generated steam in the ORC, and 16-effect parallel feed MED-MVC unit. MED-MVC was directly powered by electricity. Total solar field area was 14370 m2. TBT in the first effect was 59.9℃

15 - 4.18 0.94

28

Authors Year Location Plant Type

Energy source

System Description Feed

Water Capacity (m3/d)

GOR STEC

(kWh/m3) SEEC

(kWhe /m3) Water cost US $/m3

Sharaf et

al. [82] 2011

Egypt-Suez Gulf region

30°N

Model j

PTC

The system mainly consisted of 16-effect MED unit, solar collector field and the boiler heat exchanger (BHX) where top steam was generated with the thermal power from the heat transfer oil from solar cycle. Four different flow arrangements of MED were studied: forward (FF), backward (BF), parallel (PF), and forward feed with feed heaters (FFH).

Seawater 100

f,

BF 9.88 FF 4.48 FFH13.9 PF 15.2

BF 65.2 FF 143.8 FFH 46.3 PF 42.4

-

BF 7.14 FF 12.9

FFH 5.75 PF 5.47

The system mainly consisted of 16-effect MED unit, solar collector field, BHX and organic Rankine cycle (ORC). The exhaust steam from ORC was used as top steam of the MED unit. Water and electricity was produced at the same time. Four different flow arrangements of MED were studied.

BF 9.21 FF 4.56 FFH15.0 PF 19.4

BF 71.1 FF 143.5 FFH 43.7 PF 33.7

-

BF 8.03 FF 13.8

FFH 5.13 PF 5.06

Leblanc et

al. [86] 2010

Melbourne, Australia 37°48’N

Pilot plant Solar Pond

3-effect forward feed MED system with submerged tube evaporator, thermal energy supplied by 720m2 solar pond. The seawater in the first effect was heated up by the hot brine from the low conductive zone (LCZ). LCZ brine temperature 60~85℃.

Seawater 2.3 2.7 f

244 - 18~22

Alarcon-Padilla et

al. [87] 2010

Almeria, Spain

36°50’N

Plant Design

PTC and gas boiler

The system mainly consisted of a forward feed 14 effect MED unit, a LiBr-H2O double-effect absorption heat pump (DEAHP), the gas boiler, the PTC solar collector field with thermal storage and steam generator. Thermal energy is supplied to the MED-DEAHP system at a generator (separation of water steam from LiBr solution) of DEAHP. Gas boiler was used as the backup for the solar thermal system to guarantee 24h operation. The MED unit was connected to DEAHP indirectly via two auxiliary tanks. MED top brine temperature was designed at 70℃.

Seawater 72 >9 <72e - -

29

a. GOR (gain-output ratio) in this table is defined as the energy ratio of total latent heat of distillate to the directly input thermal energy to drive MED process (sometimes referred to as performance ratio in literature). Normally it is the latent heat of heating steam that enters the first effect that drives the MED process. At this case, GOR is equal to the ratio of distillate mass flow rate to the heating steam mass flow rate.

b. STEC (specific thermal energy consumption) refers to the amount of input thermal energy consumed per m3 fresh water production. STEC can be calculated with GOR value when these two factors refer to the same thermal desalination process. In that case, the relationship between STEC and GOR has been shown in equation (3).

c. SEEC (specific electrical energy consumption) refers to the amount of electrical energy consumed per m3 fresh water production.

d. This is calculated by assuming 8h operation per day based on the product water flow rate derived from literature [75].

e. The STEC values are calculated with the available GOR values, ∆��,+ at 60℃ (2358 kJ/kg) was used in the calculation.

f. The GOR values are calculated with the available STEC values, ∆��,+ at 60℃ (2358 kJ/kg) was used in the calculation.

g. The water cost values was converted from Chinese Yuan (26 Yuan/m3) to US dollar with an exchange rate of 6.365 (the exchange rate found in the Internet at the article received date ).

h. A steady-state CSP-MED model was used, which is established based on the SAM physical trough model from NREL (U.S. National Renewable Energy Laboratory) .

i. The MED plants were Designed and simulated using the software IPSEpro developed by SimTech Simulation Technology, Austria.

j. Design and simulation of the solar desalination plants was based on an established SDS (Solar Desalination System) software package.

Authors Year Location Plant Type

Energy source

System Description Feed

Water Capacity (m3/d)

GOR STEC

(kWh/m3) SEEC

(kWhe /m3) Water cost US $/m3

Alarcon-Padilla et

al. [76] 2008

Almeria, Spain

36°50’N

Pilot Plant

Compound parabolic

concentrator (CPC) and gas boiler

The system mainly consisted of a forward feed 14 effect MED unit, a LiBr-H2O double-effect absorption heat pump (DEAHP), the gas boiler, 500m2 stationary CPC solar collector field with 24 m3 thermal storage. Gas boiler was used as the backup for the solar thermal system to guarantee 24h operation.

Seawater 67 10 64 - -

30

2.2.4 Solar-powered MD

Membrane distillation (MD) is a thermally driven membrane separation process, in which only water

vapour or other volatile species are allowed to pass through the hydrophobic membrane. The driving

force of MD is the transmembrane vapour pressure difference. There are four basic configurations of

MD according to the different means to recover vapour in the permeate side[88]: direct contact

membrane distillation (DCMD), air gap membrane distillation (AGMD), vacuum membrane

distillation (VMD) and sweeping gas membrane distillation (SGMD). Besides, some new

configurations have also been developed by researchers in recent years, including V-MEMD [89-91]

and PGMD (a variation of AGMD) [92]. The performance of MD is largely affected by the

configuration adopted, the design of membrane module, the properties of used membrane (such as

membrane pore size distribution, porosity, thickness, etc.) and operating conditions (such as

temperature of feed water and cooling water, feed and cooling flow rate, permeate side pressure for

VMD, air gap thickness for AGMD, etc.) [88]. Modest operating temperature is required in MD

process, normally between 60~80°C [93], which enables it to be easily coupled with low-grade and

renewable energies. Solar powered membrane distillation has received considerable attention from

researchers. A schematic diagram of solar powered membrane distillation is shown in Fig.8 with

AGMD module as an example. Several demonstration plants have been built over the past decades.

Table 5 is a summary of selected solar MD studies in recent years.

Fig.8 Schematic diagram of a solar powered AGMD system

31

Several solar driven MD pilot plants using AGMD spiral-wound membrane modules have been

installed in Spain, Jordan and Egypt within a European Commission funded project ‘SMADES’ [94-

98]. One of the plant built in Jordan successfully desalinated real seawater from the Red sea with

actual output of 0.14~0.8 m3/d and specific energy consumption of 200~300 kWh/m3 [97]. Achmad et

al. [99] developed and studied the performance of a standalone 100 L/d solar driven pilot membrane

distillation system in Saudi Arabia. A novel memsys Vacuum Multi-effect Membrane Distillation (V-

MEMD) module was adopted in the system. Meanwhile a heat pump was integrated into the system to

improve the performance by simultaneously preheating the feed and cool the cooling water for

condenser. A daily production rate range of 32.4 L/d to 99.6 L/d was achieved at different weather

conditions. It was found that the presence of heat pump significantly affected the condenser input

temperature and feed temperature and considerably increased distillate flux. Raluy et al. [100]

reported the 5-year operational experience and data analysis of a 100 L/d solar MD demonstration

plant located in Gran Canary Island, Spain. During the five years operation, daily water production in

the range of 5~120 L/d with the lowest production in winter months was obtained, while high quality

distillate with conductivity at 20~200µS/cm was produced. Specific thermal energy consumption of

140~350 kWh/m3 was achieved. This plant further demonstrated the technical feasibility of

autonomous solar powered membrane distillation system. The increase of unit productivity and

reduction of specific energy consumption should be the aim of further development as pointed out by

the author. There are very limited reports on the economic evaluation of solar MD desalination. Banat

et al. [101] conducted economic analyses of small scale stand-alone solar powered MD plants based

on 0.1m3/d and 0.5m3/d plants installed in Jordan. The annual cost was calculated as a sum of

amortized capital cost, operating and maintenance cost and membrane replacement cost. Detailed

capital investment was given for each of the plant components. The potable water production cost was

estimated at $15/m3 and $18/m3 for the 0.1m3/d and 0.5m3/d plants, respectively with the distilled

water blended with 1000mg/L raw brackish water by ratio 1:1. Saffarini et al. [102] also carried out

an economic evaluation on solar powered membrane distillation systems. Water production costs from

seawater were calculated at fixed membrane module parameters and a fixed recovery ratio of 4.4%

(indicating a capacity of 158kg/h) for 3 different configurations. The cost of $12.7, $18.26 and $16.2

were estimated for solar powered DCMD, AGMD and VMD systems, respectively. It was suggested

that the cost of solar thermal collectors can account for 70~80% of the total water production cost.

As shown in Table 5, the reported GOR values of the membrane module in literature are in a wide

range of 0.7~6 with the corresponding STEC at 100~896 kWh/m3. The reported highest GOR value

corresponds to a spiral-wound AGMD membrane module developed by Fraunhofer ISE [95],

Germany, while the lowest value corresponds to a lab scale DCMD membrane module [103]. Energy

efficiency of 5 pre-commercial membrane modules have been analysed and compared in a recent

research conducted by Zaragoza et al. [104]. The specific energy consumption varies between

32

different configurations and at different operating conditions. The spiral–wound PGMD membrane

module developed by solar spring shows the lowest STEC of 210 kWh/m3, while the others vary from

200 to up to 1000 kWh/m3. This also indicates that membrane distillation process is still relatively less

energy efficient compared to mature thermal desalination technologies MED and MSF. Besides,

relatively low permeate flux of MD membrane, as well as the fouling and wetting phenomenon during

long term operation, are obstacles that interfere with the commercialization of MD. Nevertheless, MD

technology is still under development, tremendous efforts are being made to overcome these

drawbacks by using novel membrane materials, or by optimizing MD configurations and modules.

Meanwhile, further application and research also needs to be done to examine the technical and

economic feasibility of solar MD.

33

Table 5 Summary of selected solar MD studies

Authors year location Plant Type Energy source

System Description Feed water

Solar Radiation (W/m2)

Capacity (m3/d)

Permeate flux

L/m2 h

STEC (kWh/m3)

GOR GORso SSEC

kWh/m3

Shim et al.

[103] 2015

Suncheon, Korea

34°57’N Lab scale

Solar water heater and electricity

DCMD membrane module with 0.06m2 PTFE flat sheet membrane; solar heater area 4.7m2; feed water heated via a titanium heat exchanger. Inlet temperature was controlled at constant value 65 ℃ , cooling temperature controlled at 25℃. Feed flow rate 2.5 L/min.

Seawater - 0.014 g

28.5 i 896

j 0.7

j - -

Wang et al.

[105] 2014

Xiamen, China

24°28’N Lab scale

Evacuated tube

collectors (ETC)

Cross-flow rectangular hollow fibre VMD module with membrane area of 0.25 m2 ; Evacuated tube collectors with aperture area of 2.16 m2 and a 0.5 m3 hot water tank; feed water was heated via a titanium plate heat exchange.

35g/L NaCl

solution - 0.008

g

4 h

750 0.84

l - -

Chafidz et al.

[99] 2014

Riyadh, Saudi Arabia

24°38’N

Pilot Plant ETC

and PV panels

memsys V-MEMD membrane unit with 0.2µm pore size PTFE membrane ; A heat pump unit was used to cool down cooling water and preheat feed water ; ETC with CPC reflectors (CPC 1506) used as solar thermal collector, total aperture area 18m2; PV panels with peak power of 3.36 kW used to supply electricity.

Brackish water

645 e

(8am ~4pm)

0.1 e

- - - 0.71 m

929 m

Kim et al.

[106] 2013

Saudi Arabia

Mathematical

model

ETC and external

heat

50 shell-and –tube type hollow fibre DCMD modules; 3360 m2 ETC with 160 m3 water storage tank. Plate heat exchangers used to obtain heat from solar collector and also to recover heat from brine and permeate; 24 hours operation with both solar energy and other external heat supply, annual solar fraction 0.77.

Seawater - 31 f

51.14 i

436 1.50l - -

34

Authors year location Plant Type Energy source

System Description Feed water

Solar Radiation (W/m2)

Capacity (m3/d)

Permeate flux

L/m2 h

STEC (kWh/m3)

GOR GORso SSEC

kWh/m3

Gabsi et al.

[107] 2013 Tunisia Pilot plant

Flat plate collector

(FPC) and PV panels

VMD system with 5 m2 hollow fibre PVDF module; 51m2 FPC; flat plate heat exchanger used to obtain heat from solar collector; PV panels with peak power 1.5kW and batteries were used to supply power for pumps.

Seawater - 0.21 e

5.25 h

- - - -

Frikha et al.

[108] 2013

Mahare, Tunisia 34°32’N

Mathematical

model

FPC and PV panels

DCMD system with 4 m2 PVDF hollow fibre membrane module; solar collector with total aperture area of 70m2; titanium plate heat exchanger used to obtain heat from solar collector; PV modules with peak power of 2.1KW used to supply electricity.

Seawater - 0.75 e

14 h

- - - -

Abdallah et

al. [109] 2013 Tunisia

Plant Design

FPC and PV panels

VMD system with 4m2 hollow fibre PVDF membrane module; 35 flat plate solar collector, each with 2m2 aperture area;16 m2 PV field used to supply 2.1 KW electricity energy.

Seawater 700 f

0.8 f

25 h

- - - -

Raluy et al.

[100] 2012

Gran Canary Island, Spain

27°58’N

Pilot Plant FPC

and PV panels

PGMD system with 10 m2 module 6.96 m2 flat plate solar collectors; PV system with peak power of 80W for electricity supply

seawater - 0.12 e

1.5 h

140~500 k

0.5

k

~5.1 - -

Guillén-Burrieza et

al. [110] 2011

Almeria, Spain

36°50’N Pilot Plant

Compound Parabolic Collector

(CPC)

Flat sheet AGMD module, with total membrane surface area of 2.8 m2 ; 500m2 CPC with 24 m3 thermal storage system (designed for former solar MED project)

Marine salt

solution - 0.15

g 6.5

i 810 0.79 - -

Banat et al.

[94] 2010

Aqaba, Jordan

29°31’N Pilot Plant

FPC and PV panels

Four spiral-wound AGMD membrane modules with membrane area of 10 m2 each; 72m2 FPC 3 m3 heat storage tank; a titanium heat exchanger used to transfer heat from the collector to seawater; power supplied from 12 PV modules with total peak power of 1.44 kW.

Seawater - 0.9

f 2.8

h - - - -

35

a. GORso (gain-output ratio-solar) in this table is defined as the energy ratio of total latent heat of distillate to the total input solar energy to the solar MD system. It can be determined by the equation (1). Aa in the equation refers to total aperture area of solar collectors.

b. SSEC (specific solar energy consumption) refers to the amount of solar energy (that reached the aperture area of solar thermal collectors) consumed per m3 fresh water production. The relationship between SSEC and GORso is shown by equation (2). ∆�� = 2360 kJ/kg will be used for calculation of GORso and SSEC when the values are not available in literature. All SSEC and GORso values in this table were calculated by the authors.

c. GOR in this table refers to the gain output ratio of the membrane module, which is defined as the energy ratio of total latent heat of distillate to the total input thermal energy into the membrane distillation system.

It can be calculated with the following equation for a membrane module without internal heat recovery.

GOR =��∆��

�/01(+23+') (4)

��, average mass flow rate of distilled water, kg/h; ∆�� , latent heat for evaporation, kJ/kg; �4, membrane feed mass flow rate, kg/h; Cp heat capacity of water, kJ/kg ℃;

Ti and To refers to feed flow temperature at inlet and outlet of the membrane module, respectively.

Authors year location Plant Type Energy source

System Description Feed water

Solar Radiation (W/m2)

Capacity (m3/d)

Permeate flux

L/m2 h

STEC (kWh/m3)

GOR GORso SSEC

kWh/m3

Dow et

al.[111] 2010

Edenhope, Australia 37°03’S

Pilot Plant ETC

DCMD system with total membrane area of 1.4m2; 8 ETC collector with a total aperture area of 18m2 used to heat up the feed water.

RO brines from

brackish water

desalination plant

- 0.12 f

7.14 i

- - - -

Wang et al.

[112] 2009

Hang Zhou, China

30°15’N

Lab Scale FPC

VMD system with hollow fibre membrane module with total membrane area of 0.09m2; 8 m2 FPC to supply thermal energy.

Ground-water

- 0.016 e

29.7 i - - - -

Koschikow ski et al. [95]

2009

Gran Canary Island, Spain

27°58’N

Pilot Plant FPC

and PV panels

5 AGMD membrane modules with internal heat recovery; 90m2 FPC used to supply thermal energy; PV modules with 1.92kW peak power to supply electricity.

Seawater - 1.6 f

- 100~200 3~6 - -

Fath et al.

[96] 2008

Alexandria, Egypt 31°12’N

Pilot Plant FPC and PV

panels

AGMD spiral-wound membrane module ;Three corrosion-free solar collectors with a total aperture area of 5.73m2 connected in series used to supply thermal energy

-

604 e

(7am ~7pm)

0.064 e

- - - 0.9 728 m

Banat et al.

[98] 2007

Irbid, Jordan

32°33’N Pilot Plant

FPC and PV panels

AGMD membrane module with effective area of 10m2; 5.73m2 corrosion-free solar collector used to heat the feed directly; PV modules with peak power of 850W used to supply electricity.

- 875 e

0.11 e

2.5 i

200~300 2.2 l

~3.3

1~2

327~655

m

36

d. STEC (specific thermal energy consumption) refers to the amount of directly input thermal energy the MD system consumed per m3 fresh water production. STEC can be calculated with GOR value with equation (3).

e. In these reference, the solar radiations refers to daily average values between certain period of time when data is available or average value by assuming 8h daylight time. The capacity of plant refers to daily production on a specific test day (maximum daily values were selected).

f. Capacity and solar radiation values in theses reference are all based on designed condition. 8h operation was assumed when only designed mass flow rate was given.

g. Capacity values in these reference are calculated based on the permeate flux values by assuming 8h operation.

h. The permeate flux values in these reference are daily average values based on the capacity. 8h operation was assumed when actual operating time was not available.

i. The permeate flux values in these reference are maximum flux observed during the operation or flux at optimized experimental conditions.

j. The STEC and GOR values are the reported minimum energy consumption and highest GOR under optimized experimental conditions.

k. These are the range of daily average values during the long term operation between the year 2009 and 2010.

l. The GOR values are calculated with the available STEC values, ∆��,+ at 60℃ (2358 kJ/kg) was used in the calculation.

m. The GOR values or SSEC values are calculated based on equation (1) or (2), ∆��,+ at 60℃ (2358 kJ/kg) was used in the calculation.

37

2.2.5 Solar-powered RO

Reverse Osmosis is regarded by far the most efficient desalination technology due to its low specific

energy consumption (2~5 kWhe/m3) while bearing the largest desalination installation capacities with

around 65% share in the world [64]. In RO, saline water is fed to the membrane with high pressure to

overcome the osmotic pressure of the feed water. Freshwater is collected at the permeate side while

the concentrated brine is rejected. Feed pressure of RO systems usually range from 6~8 MPa for

seawater applications and 0.6~3MPa for brackish water applications [113]. The productivity and

recovery ratio of RO is largely affected by the properties of used membrane, salinity of feed water,

feed pressure and feed flow rate, etc. The well-established energy recovery devices in modern RO

plants contributed significantly to the high energy efficiency of the process. Much work and research

have been conducted in terms of solar driven RO desalination systems. When combined with solar

energy, RO can be either driven by electrical power that is generated by photovoltaic (PV) cells or

solar thermal power plants, or by mechanical power that is converted from solar thermal energy by

thermodynamic cycle. Lists of some PV driven RO studies and solar thermal driven RO studies in

recent years are given in Table 6 and Table 7, respectively.

Fig. 9 Schematic diagram of PV-driven RO system

In a PV powered RO system, PV and RO subsystems works independently. Since both of them are

mature commercialized technologies, technical feasibility of PV-RO plant is not much of an issue

compared to its economic feasibility and reliability [13]. A considerable number of small scale or

pilot PV-RO seawater/brackish water desalination plants have been built and investigated since 1980s.

A comprehensive list of PV driven RO plants (1980~2008) was summarized by Ali et al. [65]. Thus

38

Table 6 just lists selected PV-RO plants and studies reported from 2009. Recently, Peñate et al. [114]

reported the seven year uninterrupted operation (2006~2013) of a standalone PV driven RO plant in

Tunisia aiming to supply freshwater for a 300 inhabitant village located in the Sahara desert. The

plant successfully produced more than 15 million litres of freshwater with TDS lower than 300 mg/L

from brackish groundwater (TDS 4.0~4.5g/L) at monthly average production rate in the range of 3.26

~12.8 m3/d. The specific energy consumption was in the range of 1.64~3.13 kWh/m3. Qiblawey et al.

[115] investigated the performance of a standalone PV-RO pilot plant in Jordan. When RO operating

pressure was set at 0.6 MPa, the specific energy consumption was found to be 16 kWh/m3 and

recovery ratio was 54%. Economic evaluation for the plant was presented in an earlier study [116].

Water cost of the PV-RO system was estimated as $15.6 /m3 when membrane lifetime was assumed at

5 years. Reported water cost for small scale PV driven RO plants in previous literature are normally in

the range of $7~30/m3 [116]. A much lower water cost of $0.825 /m3 was estimated by Alsheghri et al.

[117] probably because PV system will be built with much larger capacity than RO requires to sell

electricity to grid in the proposed design.

Unlike PV driven RO system which is relatively mature and commercially available in small scale,

the research and studies on thermal driven RO system are mostly in modelling or laboratory stage.

Solar thermal driven RO system are mostly based on the organic Rankine cycle (ORC) technology

which is commonly adopted in solar thermal power generation systems. The ORC is a thermodynamic

power cycle that uses organic working fluid and converts the thermal energy input to the output

mechanical energy. An ORC normally consists of pressure pump, evaporator, turbine and condenser

[116]. Organic working fluid is pressurized and injected into the evaporator where it is heated up to

evaporate by heat exchange with high temperature fluid. And then the vapour expands through a

turbine to produce mechanical work to drive the high pressure pump for RO. Meanwhile, the feed

saline water could be preheated in the ORC condenser to increase the RO membrane permeability.

Schematic diagram of solar ORC driven RO system is shown in Fig.10.

Delgado-Torres et al. [118] has proposed general design recommendations for solar ORC driven RO

system. Information about selection of the working fluid and boundary conditions of ORC, operating

temperature, suitable solar collector and configurations of solar field, etc. are present in detail.

Concentrating solar collectors such as parabolic trough collector, linear Fresnel concentrators, and

compound parabolic concentrators were recommended for solar ORC to maximise the overall

efficiency. The organic working fluid used in ORC should be selected to match the level of heat

source temperature achieved with different solar collectors. Siloxane MM was suggested for ORC

driven by parabolic trough collectors and other working substances such as Solkatherm® SES36,

isobutene, isopentane, R245ca and R245fa were suggested for ORC driving by stationary solar

collectors [13]. Xia et al. [119] simulated a wind-solar ORC driven RO system by theoretical

modelling. The performance and the influence of certain parameters on fresh water production were

39

examined, including turbine inlet pressure, feed water pressure and salinity, and condenser

temperature of ORC. Water production of 1,186 m3 was achieved with the solar ORC alone in a sunny

summer day with specific solar energy consumption (SSEC) of 69 kWh/m3, which is fairly close to

the specific thermal energy consumption (STEC) of conventional fossil-fuel based thermal

desalination plants. And it was found that turbine inlet pressure and condenser temperature had

significant influence on the nett work output of ORC and fresh water production.

Fig. 10 Schematic diagram of solar ORC driven RO system

As shown in Table 6 and Table 7, reported PV-RO plants in recent years are mostly small to medium

scale with the capacity of 0.2~200 m3/d for seawater or brackish groundwater treatment. In contrast,

solar thermal RO applications are studied only for large scale seawater desalination plants with the

capacity of 1186~50,000 m3/d although no real plant has been reported. Specific energy consumption

(SEC) of RO process lies in the range of 1.2~16 kWh/m3. Water cost of PV-RO system was reported

as $0.825~15.6/m3, while the water cost of a large scale solar thermal RO plant was estimated as

$2.18/m3 [120].

Currently, the operation of PV-RO system relies largely on lead acid batteries for energy storage. The

long term performance stability and maintenance remains an issue in remote area. Cost effective

approaches for energy storage to supply RO operation should be developed for future application.

Besides, brine disposal is another big challenge for RO application in inland regions. Increasing

recovery ratio by optimization of RO process or integration with other technology like MD, or

developing zero liquid discharge schemes may be the potential solution of this issue. Solar thermal

RO is still immature at present. Further experimental or modelling studies need to be done to

demonstrate the technical and economic feasibility of this technology.

40

Table 6 Summary of selected PV driven RO studies

Authors year location Plant type Energy source

System description

Feed water Capacity (m3/d)

Work Pressure (MPa)

Recovery Ratio

%

SEC (kWh /m3)

Water cost US $/m3

Alsheghri et

al. [117] 2015

Abu Dhabi, UAE

24°28’N

Plant Design

PV /grid

720kW PV system to export electricity to the grid 1757.8 MWh/annual; RO operation directly powered by grid electricity.

Seawater 200 - 40 6.99 0.825

Penate et

al. [114] 2014

Kebili, Tunisia 33°42’N

Small Scale Plant

PV

Solar PV modules with peak power of 10.5 kW and a 660Ah battery bank. Pretreatment of RO consisted of a sand filter, chemical dosing, an activated carbon filter and a cartridge filter. Brine was used for irrigation after mixing with raw water.

Brackish ground water

50 1.24~1.38 70 1.64~3.13 -

Qiblawey

et al. [115] 2011

Hartha Village, Jordan

32°33’N

Pilot Plant PV

PV module with peak power output of 433W and 460 Ah battery pack. Pretreatment of RO consisted of softener, 5-micron sediment filter, granular activated carbon filter (GAC) and 1-micron sediment filter.

Brackish ground water

0.5 0.6 54 16 15.6

Khayet et

al. [121] 2010

Madrid, Spain

40°23’N Pilot Plant PV

The RO plant was coupled to a solar heat spherical collector and PV panels. Tubular RO membrane module with an effective surface area of 1.2m2 was used.

Brackish water with TDS 6g/L

0.2 0.77 - 1.2~1.3 -

Munari et

al. [122] 2009

Cooper Pedy,

Australia 29°0’S

Pilot Plant PV

UF-NF/RO hybrid membrane system; PV modules with peak power output of 300W without batteries; system operated at variable flow and pressure.

Brackish Groundwate

r 1 0~1.2 10 3~5.5 -

41

Note:

a. SEC (specific energy consumption) in this table refers to the amount of electrical energy consumed by per m3 fresh water production in the reverse osmosis process.

Table 7 Summary of selected solar thermal driven RO studies

Note:

a. SEC (specific energy consumption) in this table refers to the amount of energy consumed by per m3 fresh water production in the reverse osmosis process. It can be in the form of electrical energy or mechanical energy when the high pressure pump is directly driven by electrical power or the mechanical work output by ORC, respectively.

b. SSEC (specific solar energy consumption) refers to the amount of solar energy (that reached the aperture area of solar thermal collectors) consumed per m3 fresh water production.

c. This is daily average value by assuming 10h operation per day.

d. The water cost is converted from Euro to US dollars with exchange rate 1.4 (historical value for the article received date).

Authors year location Plant type Energy source

System description

Feed water

Solar Radiation

W/m2

Capacity (m3/d)

Work Pressure (MPa)

Recovery Ratio

%

SEC (kWh /m3)

SSEC (kWh /m3)

Water cost US $/m3

Xia et al.

[119] 2015

Qingdao, China

36°04’N

Mathematical Model

Compound Parabolic Collectors (CPC) and

Wind Turbines

RO system with pressure recovery driven by hybrid energy source: solar powered ORC and wind energy (electricity). 210 CPC thermal collectors with aperture area of 48m2 each; 5 wind turbine with diameter of 10m each.

Seawater 813c

1,186 (solar) 1,325 (wind)

8 - 3.2

69

Penate et al.

[123] 2012 Spain Plant Design

Parabolic trough

collector (PTC)

A solar ORC power plant used to supply electricity to drive the RO seawater desalination system. Total effective membrane area 7283m2 with an average flux of 14.30 L/m2·h; solar collector area 2099 m2.

Seawater - 2,500 6.9 40 2.99 -

Salcedo et

al.

[120] 2012

Tarragona,

Spain 41°06’N

Mathematical Model

PTC and natural

gas fired heater as backup

The desalination system included 7 RO trains each with 37 m2 spiral-wound membrane; driven by solar Rankine cycle with gas fired heater as backup. Annual solar fraction is 43.4%.

Seawater - 50,000 6.3 - 3.53 - 2.18 d

42

2.2.6 Solar-powered ED

In electrodialysis (ED) process, saline water is pumped through an ED stack where anions and cations

move toward opposite side while an electric potential difference is applied across the cathode and

anode. The ion exchange membranes between them enable the desalination of saline water and

concentration of ions in separate parts [124]. ED systems are more favourable in desalination of

brackish water with relatively low TDS as it is normally regarded as not economically competitive for

seawater desalinations due to the expensive ion exchange membranes, costly electrodes, and relatively

short lifetime while working in high-density electric field [13]. Electrodialysis Reversal (EDR) is

widely used in the application of ED, in which periodic reversal of DC electric field polarity was

conducted to reduce membrane scaling and fouling. In ED, the water quality after treatment and the

productivity will be affected by feed water salinity, retention time, DC voltage and the time interval

between polarity changes in EDR, etc [125]. As an electricity-driven process, ED is suitable to be

combined with PV system.

Fig.11 Schematic diagram of PV-driven ED system

Table 8 provides a summary of selected solar ED studies in recent years. Although the largest PV-ED

plant reported in the 1990s had distillate production of 200 m3/d [65], studies reported in recent years

are mostly laboratory scale batch studies or small scale plants with less than 50 m3/d capacity. In

terms of renewable energy driven desalination, ED is highly valued for its adaptability for variable

energy conditions as ED could work at a wide range of DC voltages. Thus ED directly powered by

PV has received researchers’ interests. Peñate et al. [125] reported the performance tests of a PV-

driven ED plant with a nominal production flow of 4 m3/h. A PV field with battery and DC/AC

43

inverter was used for power supply of pumps, valves and control unit, etc. while the EDR unit was

directly powered by two other PV fields under variable conditions without battery. The operation of

the EDR plant was controlled by a PLC controller which could change between 5 different PV field

modulation modes to obtain favourable power supply at a wide range of solar radiations. In a 5h

automatic operation during a sunny day, 16.9 m3 water with 1050 µS/cm average conductivity was

produced from 5300 µS/cm feed brackish water. The specific energy consumption (SEC) of the EDR

unit was 0.79 kWh/m3. One of the major draw backs of ED is its inefficiency in removing organic

compounds. Only a small amount of charged organic compounds will be removed while most

uncharged organic matters will stay in the water, which make it less favorable for desalination of

contaminated source water with organic pollutants. In a recent study, Zhang et al. [126] proposed a

PV powered hybrid system of forward osmosis (FO) and ED for brackish and wastewater treatment,

in which feed water first went through the FO membrane to remove the contaminants and then ED

was used for desalination of draw solution (NaCl solution). Reasonable TOC and salt removal

efficiency was achieved in the produced water which met potable water standard in general. An

economic analysis was done for a small potable water production system with 130 L/d capacity. The

water production cost was estimated at 3.32~4.92 Euro /m3 (about USD 4.42~6.54). Economic

evaluation has also been done by Ortiz [127] for a 15m3/d PV-driven ED plant for brackish water

(2300~5100mg/L) desalination based on the result from laboratory scale demonstration. The cost of

produced water was in the range of 0.14~0.23 Euro/m3 (about USD 0.20~0.34) for irrigation purpose

and 0.17~0.32 Euro/m3 (about USD 0.26~0.48) for drinking purpose.

In terms of brackish water treatment, as shown in Table 8, the reported SEC of ED is 0.72~4.05

kWh/m3, which lies in the range 0.4~4 kWh/m3 summarized by Fernandez-Gonzalez et al [128]. The

energy consumption is even lower than RO for brackish water with low salinity, i.e. TDS<5000mg/L.

Accordingly, PV-ED should be economically competitive with PV-RO. Water production cost from

brackish groundwater under 5000mg/L TDS have been reported in the range of $0.19~16.0/m3 [128]

with lower bound less than that of PV-RO (see section 2.2.5). The potential of ED in running directly

with PV arrays is a particular advantage over RO. Less reliance on batteries results in less capital cost

of the PV system. Thus, the research on battery free ED system should be a future focus. The

capability of PV-ED system in operating with variable feed water quality and weather conditions

should be further investigated [124].

44

Table 8 Summary of selected PV driven ED studies

Note: a. SEC (specific energy consumption) in this table refers to the amount of electrical energy consumed per m3 fresh water production. b. The cost was estimated for a 130 L/d plant based on optimized operational condition examined in the lab scale study. The value was converted from Euro with an exchange rate of 1.33 (exchange rate at paper received time obtained from Internet). c. The cost was estimated for a 15m3/d plant based on mathematical simulation. The value was converted from Euro with an exchange rate of 1.50 (exchange rate at paper received time obtained from Internet). d. This refers to the fresh water production for a specific testing day.

Authors year location Plant type

System description

Feed water Capacity

(m3/d) SEC

(kWh/m3) Water cost US $/m3

Zhang et al.

[126] 2013

Heverlee,Belgium 50°51’N

Lab scale

FO-ED hybrid membrane system; feed water first come through FO with NaCl as draw solute, then ED is used to desalinate the NaCl solution. ED stack consisted of 5 cell pairs with total effective surface area of 0.029m2. PV panels with 0.0648m2 surface area and 8~9 V voltage output used as power supply

Wastewater treatment plant effluent

Batch test - 4.42~6.54b

Penate et al.

[125] 2013

Gran Canaria island, Spain

27°58’N

Small plant

EDR unit with 340 cell pairs, 2 electrical stages and a nominal product flow of 4m3/h. A PV field with battery and 5.8kW peak power was used to power pumps, valves and PLC control unit, etc. 2 other batteryless PV fields with 2.45kW and 1.24kW peak power were used to power two different electrical stages of EDR pack, respectively. A PLC control unit was used to change the operational mode according to solar radiation.

Brackish water with conductivity of

5300 µS/cm 16.9

d 0.79 -

Cirez et

al.

[129] 2013 Spain

Lab scale

ED unit with 10 cell pairs and total effective membrane area of 0.2m2; special designed PV module providing open circuit voltage up to 13.7V and peak power up to 30W.

Simulated brackish water with 5000mg/L

NaCl solutions Batch test 4.05 -

Ortiz et al.

[127] 2008

Alicante, Spain

38°24’N

Lab scale

ED unit with 70 cells and total effective membrane area of 3.5m2. PV panels with peak power of 154W

Brackish groundwater with TDS

2300~5100mg/L Batch test

0.92~1.69 drinking; 0.72~1.43 irrigation

c 0.26~0.48

drinking; 0.20~0.34 irrigation

AlMadani et al.

[130] 2003

Isa Town, Bahrain 26°10’N

Lab scale

ED unit consisted of 24 ionic cell pairs arranged with two electrical stages; 4 PV panels with total peak power of 132 W.

Brackish groundwater with 3300mg/L

0.19~1.14 - -

45

2.3 Summary of solar desalination technologies

Fig.12 is a summary of potential combinations of solar technologies and desalination processes.

Among all the indirect desalination technologies, RO and ED could be directly powered by electricity

from PV system or CSP plant. Solar thermal collectors are used to drive other indirect solar

desalination processes HDH, MD, MSF and MED after converting solar radiation into thermal energy.

Meanwhile, mechanical energy that drives the high pressure pump in RO process can also be

indirectly derived from solar thermal collectors through solar organic Rankine cycle (SORC). Heat

pumps (TVC, AHP, ADHP, MVC) are generally adopted to enhance the energy efficiency of MED

process. These heat pumps are normally driven by thermal energy from the solar thermal collectors

with MVC as an exception which could be powered by the electricity from PV or CSP plant. The

auxiliary equipment used in those solar thermal desalination processes such as circulation pumps also

can be powered by the electricity from PV or CSP plant. The integration of CSP plant and solar

desalination has been proposed for large scale applications, where CSP either export electricity to

drive RO, ED, MVC or export exhaust/motive steam that can be used to power MED and MSF

process.

Fig.12 Potential integration of solar technologies and desalination processes

Michael et al. [131] have defined four capacity ranges for renewable energy powered desalination

plants based on the requirement of different types of markets. These ranges are: very small scale

plants (<1 m3/d) targeting at end-users such as households, families living in isolated areas; small

scale plants (<10 m3/d) which could provide the daily water needs for more than 100 people, suitable

46

for small villages, small islands or hotels; medium scale plants (10~1000 m3/d) that can supply water

for large users like towns, villages in water stressed areas; and large scale plants (>1000 m3/d) which

can be used for municipal water supply. As shown in Table 9, the various solar desalination

technologies discussed above are favourable for different capacity applications. Reported water costs

of these technologies at different plant scales are also shown in the Table. According to Ioannis et al.

[132], water cost for small desalination plants using conventional energy source ranges between

$0.2~1.3/m3 for brackish water desalination and between $0.4~3.4/m3 for seawater desalination.

Compared to this, the water cost for small to medium scale solar desalination plants are quite high. On

the other hand, the estimated water costs for large scale plants could be comparable with the

conventional large scale commercial desalination processes, which were reported as $0.5~1.5/m3 for

seawater desalination [83]. However, it should be noted that the cost estimation are mainly based on

simulation as no large scale solar desalination plant has been built yet.

Table 9 Suitable plant capacities of different solar desalination technologies and their reported water

costs.

Very Small Scale Small Scale Medium Scale Large Scale

Solar Still $6.0~65.0 /m3 N/A

Solar HDH $4.4 /m3 $2.9~22.1 /m3 N/A

Solar MD $12.0~18.0 /m3 N/A N/A

Solar MSF N/A $ 1.4~1.6 /m3

Solar MED $18.0~22.0 /m3 $4.1~8.0 /m3 $0.9~1.3 /m3

PV-RO $15.6 /m3 $6.5~12.8 /m3 $0.8~8.4 /m3

PV-ED $ 0.2~16.0 /m3 $5.7~12.1 /m3

Solar Thermal RO $2.2 /m3

CSP+RO/MED/MSF $0.9~1.2 /m3 (MED)

Note:

1) The water cost values are derived by combining data collected in Table 1~8 and reference [101],

[102], [116], [128]

2) The highlighted columns in each row represent the recommended capacity of each technology.

47

3. Solar photocatalysis and disinfection

3.1 Solar water photocatalytic application

Photocatalysis is one of the most effective technology for the mineralization of refractory organic

compounds and water pathogens among AOPs (Advanced oxidation processes). Major types of

photocatalysis in terms of fundamentals include heterogeneous photocatalysis which employs

semiconductor catalysts for water treatment and homogeneous photocatalysis which mainly refers to

photo Fenton process [133].

In heterogeneous photocatalysis process, the degradation of recalcitrant organics is governed by the

combined action of a semiconductor photocatalyst, an energetic radiation source and a highly reactive

oxygen species [134]. Among those semiconductor catalysts (TiO2, ZnO, Fe2O3, CdS, GaP and ZnS)),

TiO2 has received the most interests. The Principle of heterogeneous photocatalyst of semiconductor

is often explained based on band model. An electron (e-) in an electron-filled valence band (VB) will

be excited to a vacant conduction band (CB) when the semiconductor catalyst absorbed a photon with

energy hv equal to or greater than its band gap energy [135]. Meanwhile a positive hole (h+) is left in

the VB. The photo-generated electrons and positive holes cause the reduction and oxidation of

different compounds, respectively. In the case of TiO2 photocatalysis, the photo-generated electron-

hole pair in the surface of TiO2 triggers a series of chain oxidative-reductive reactions normally in the

presence of water and dissolved oxygen [133]. Some highly reactive species formed in this process

such as OH· radicals and H2O· radicals are regarded as major oxidants, which react with the majority

of organic substances. There are basically two main configurations of heterogeneous photocatalytic

reactors based on the state of photocatalysts [136]: reactors with suspended photocatalyst particles and

reactors with immobilized photocatalysts. The first configuration needs additional separation process

for recovery of phtocatalytic particles such as sedimentation and filtration, while the second

configuration permits a continuous operation owing to the fixation of photocatalysts onto activated

carbon, mesoporous clays, fibres and membranes [133].

The most commonly used homogeneous solar photocatalytic process in water/wastewater treatment is

photo-Fenton. Fenton oxidation process is a widely used AOP technology which relies on the catalytic

reaction of H2O2 with iron ions that produces OH· radicals as the major oxidizing species (as shown

in Eq. (5)(6)), while Photo-Fenton (ph-F) process combines Fenton reagents (H2O2 and Fe2+) with

UV-vis radiation (λ＜600nm) which will increase the production of OH· radicals by additional

reactions (see Eq.(7) ( 8)) [137]. Compared to conventional Fenton oxidation, Ph-F embraces a higher

oxidation rate, less iron utilization and as a consequence less sludge generation. Both solar and UV

light can be used for Ph-F and the radiation has significant effects on inactivation of microorganisms.

H2O2 + Fe2+ Fe3+ +OH- + •OH (5)

48

Fe3++ H2O2 Fe2++ HO2•+ H+ (6)

Fe(OH)2++ hv Fe2++ •OH; λ<580nm (7)

H2O2 + hv 2•OH; λ<310nm (8)

Photoreactors are required in industrial applications of solar photocatalytic process to harness solar

radiation efficiently. The most commonly used photoreactors are [135, 138]: Parabolic Trough

Collectors (PTC), Compound Parabolic Collectors (CPC) and Inclined Plate Collectors (IPC).

Fig.13 Typical layout for 3 different photoreactors: (a) Parabolic Trough Collectors (PTC), (b)

Compound Parabolic Collectors (CPC) and (c) Inclined Plate Collectors (IPC) [138].

PTC and CPC reactors are based on the well-established concentrating solar thermal collectors. In a

PTC reactor, long, reflective parabolic surface is used to concentrate incident solar radiation on the

focal line where a transparent tube through which the reactant fluid flows is placed. In the case of

heterogeneous photocatalytic process, suspended photocatalysts are mostly investigated with PTC

reactors [138]. PTC can only capture direct sunlight which is regarded as a disadvantage. Either

single-axis or dual axis sun tracking systems should be used in PTC reactors. In CPC reactors, two

sections of parabola facing each other were used to concentrate the sunlight onto absorber tube at the

focal line. CPC can harness solar radiation more efficiently than PTC as both direct and diffuse

sunlight will be reflected onto the absorber tube. Meanwhile, incident sunlight is distributed evenly on

the entire absorber tube surface, making it suitable for use with both suspended photocatalysts and

immobilized photocatalysts [138].

Inclined plate collector (IPC) is a flat or a corrugated inclined black plate over which the reactant fluid

flows in a thin film. It is simple in construction and especially suitable for use with photocatalysts

supported on the surface of inclined plate, which have been named as ’thin film fixed bed reactor

(TFFBR)’ [139]. A very low flow rate on the bottom surface (normally 0.15~1.0 L/min) is required to

maintain the ‘thin film’ (typically 100~200µm) [135]. Thus IPC requires larger surface area than the

PTC and CPC. It is regarded as more suitable for small scale applications.

49

The influence of operating conditions on photocatalytic processes have been studied extensively by

researchers. Major influencing factors include solar radiation and weather conditions, catalyst load,

dissolved oxygen concentrations, pH, temperature, contaminants type and load, etc. [133, 135]

Lists of pilot studies of heterogeneous and photocatalytic processes reported in recent years are

summarized in Table 10 and Table 11, respectively. Special research focus has been placed on

degradation of persistent organic pollutants such as pesticides and PPCPs (Pharmaceuticals and

Protective Care Products) with heterogeneous photocatalytic process. Several pilot studies have been

conducted for treatment of municipal WWTP effluent [140, 141]. Meanwhile solar photocatalytic

processes with TiO2 have also been studied for application in drinking water disinfection, which is an

improvement from simple solar disinfection (SODIS). Some microorganisms which are resistant to

UV-A irradiation have been successfully inactivated by TiO2 photocatalysis [142]. Photo-Fenton

technology is mostly investigated for industrial wastewater treatment. Michael et al. [143]

investigated the performance of a solar photo-Fenton process in treating olive mill wastewater (OMW)

with a CPC photoreactor. Flocculation/coagulation was selected as pretreatment method to reduce

turbidity of raw water. In the optimal dosage of Fe2+, H2O2, 87% COD removal was achieved after

240 min of solar radiation while dark Fenton at same operating conditions only achieved 55%

removal. An economic evaluation has been done to assess the economic feasibility of this technology.

The investment and operational cost of a 50m3/d OMW treatment plant was estimated on a five year

basis. The wastewater treatment cost was estimated at 2.11 Euro/m3 (USD 2.9 with exchange rate of

1.373). Although solar photocatalytic technology has exhibited good results in treating a variety of

wastewater, the present research are still limited to small scale batch studies. Except for optimization

of operating conditions to achieve better performance, the design of efficient photoreactors to be used

in variable feed water and weather conditions is also of vital importance for the future scaling up.

Besides, in-situ long term experiments need to be conducted for certain applications to further

investigate the reliability of this technology [135].

50

Table 10 Summary of selected heterogeneous photocatalytic pilot studies for water/wastewater treatment

Authors Year Raw Water Capacity Major Contaminants and

concentrations Photocatalyst type

and load Reactor type Reactor/Process details Optimal Performance

Sousa et al.

[140] 2012

Municipal wastewater

treatment plant (WWTP) effluent

25L Batch

Emerging contaminants with special focus on pharmaceutical compounds DOC of 12.1mg/L

Suspended TiO2 P25,

CPC

0.91m2 CPC unit with four borosilicate tubes

19 out of 22 pharmaceutical compounds in the effluent were completely removed with total accumulated UV energy of 32KJ/L. 35~77% removal efficiency achieved for the rest three compounds.DOC removal efficiency was around 60%.

Fenoll et al.

[144] 2012

Drinking water with pesticides

spike

250L Batch

eight miscellaneous pesticides (ethoprophos, isoxaben, metalaxyl,metribuzin, pencycuron,pendimethalin, propanil and tolclofosmethyl) 65~115 µg/L

Suspended ZnO, TiO2, SnO2,WO3,

and ZnS

150mg/L

CPC 1.27m2 photoreactor

module with 8 borosillicate tubes

ZnO shows the best performance. 29~126min reaction time required for 90% degradation for the pesticides

Miranda-Garcia et al.

[141]

2011

Municipal wastewater

treatment plant (WWTP) effluent

10L Batch

emerging contaminants (acetaminophen, antipyrine, atrazine, carbamazepine, diclofenac, flumequine, hydroxybiphenyl, ibuprofen, isoproturon, ketorolac, ofloxacin, progesterone, sulfamethoxazole and triclosan) DOC 13mg/L

Immobilized TiO2 P25 supported by borosilicate glass

spheres synthesized by sol-gel

CPC

Two CPC modules with total illumination area of 0.30m2 and twelve Pyrex glass tube mounted on a fixed platform tilted as

local latitude

85%compounds degraded within 120min of illumination time

Zayani et

al.

[145] 2009

Commercial azo dye solution

2 m3 batch Yellow Cibacron FN-2R (YC) TOC 30mg/L

Immobilized TiO2 P25 10g/m2

IPC

Thin-film fixed bed reactor with an area of 25m2, 20 degree inclined angle. Flow rate 3 m3/h

Up to 80% removal of TOC achieved after 8h

treatment (a day)

51

Table 11 Summary of selected photo- Fenton pilot studies for wastewater treatment

Authors Year Raw Water Capacity

Major Contaminant and concentrations

Reactor Type

Operational Conditions

Optimal Performance H2O2 dosage

mg/L Fe2+ dosage

mg/L Initial pH

Velegraki et al.

[146] 2015

Winery wastewater

200L batch mode

Organic matters including soluble sugars, organic acids, alcohols and high-molecular-weight compounds, such as esters, polyphenols, tannins and lignin Initial COD about 1200mg/L Initial DOC 435 mg C/L

CPC

Initial 500; stepwise additions

to maintain at 100~500

5 2.8

80% COD and DOC after 402min reaction time with 63KJ/L accumulated UV energy

Michael et al.

[143] 2014

Olive mill wastewater

(coagulation /flocculation

used as pretreatment for removing

suspended solids )

100L batch mode

Organic wastewater with high levels of phenolic compounds, Diluted sample used for photo-Fenton study Initial COD 350mg/L

CPC 1000 80 2.8~2.9 COD removal 87.3% after 240min irradiation

Hernandez-Rodriguez et al.

[147] 2014

Synthetic wool dying

wastewater with dye solution

5L Batch

Different dye solutions (Yellow, Blue and Red) with COD 2089~2388mg/L TOC 973~1070 mg/L

CPC 4540~6050 120~240 3.0

75% ~79% TOC removal after 139min illumination time with 20KJ/L accumulated energy

52

3.2 Solar Disinfection of water (SODIS)

Fig.14 Schematic diagram of SODIS

Solar disinfection (SODIS) of drinking water is a simple, low-cost household water treatment

technology promoted by WHO (World Health Organization) which is suitable for developing

countries that lacks access to safe drinking water supply but receives abundant solar radiation. SODIS

only requires that water is stored in transparent containers (usually PET bottles) in which they are

exposed to direct sunlight for continuous periods to enable waterborne pathogens be inactivated by

sunlight [148]. In SODIS, both thermal and optical inactivation (with UV radiation) occurs and they

have a strong synergistic effect at temperatures over 45℃ [149]. Considerable studies and field works

have been done on SODIS since 1880s. The efficacy of SODIS has been well proved. Being effective

against almost all waterborne microbial species, SODIS can significantly reduce rates of childhood

dysentery and infant diarrhoea by 45% as indicated by clinical trials [150]. Factors that affect the

SODIS process are widely investigated by researchers, including received UV radiation, bottle

properties, water quality, water temperature, and the external configurations, etc. [151]. The cost of

SODIS has been estimated as $0.63 per person per year by Clasen et al. [152], being the lowest

household based disinfection intervention when compared with chlorination, filtration,

flocculation/disinfection, etc. Currently, SODIS is used by more than 4.5 million people for drinking

water disinfection in more than 50 countries all over the world [150]. The future research of SODIS

should be focused on enhancing the performance with physical or chemical approaches that utilize

locally available resources and better not involve additional cost, as well as promoting the technology

among local people with acceptable educational effort.

53

4. Conclusions

In the context of global water scarcity and future energy crisis, water treatment technologies driven by

solar energy are sustainable alternatives to address the worldwide water problem and reduce the

harmful impact of burning fossil fuels. The selection of solar water technologies can be very site

specific. Among the various technologies, there are simple, low-tech, low investment technologies

such as solar still and SODIS, which is especially suitable for remote regions in developing countries

that lack abundant financial support and access to high technology and skilled workers. Although

some of them are not commercially available, indirect desalination technologies and solar

photocatalysis technologies are becoming more reliable and technically mature with the developments

and technical improvement in both solar technologies and the water treatment processes. Compared to

conventional fossil-fuel based desalination plants, water production cost from solar based desalination

processes are still relatively high largely due to the costly solar collectors, which is a major reason

that restricted the commercialization speed. However, in most cases, the environmental cost of using

non-renewable energy has been awfully ignored. The depletion of fossil fuel energy, the emission of

greenhouse gases and air pollution should all be taken into consideration in the energy market in order

to have a sustainable future. Besides, estimated costs of large scale solar desalination plants showed

that they could be economically comparable with conventional plants although no real large scale

plant has been built yet. Furthermore, there is still much room for price decline of solar collectors with

the development of solar technologies. Solar based desalination will be more competitive in the near

future.

In terms of the current research on different solar water treatment technologies, a serious issue is,

most researchers speak highly of the technology of their own focus while ignoring the limitations and

problems exists in the area. This makes it even difficult to evaluate and compare different

technologies. Meanwhile, most technologies are still under research and development, therefore very

rare studies are based on real plants. Further demonstration and modelling studies need to be

conducted for most of the processes for scaling up and commercialization purpose. Furthermore, two

other common economic-technical issues for most of the technologies include: 1) how to match the

intermittence of solar energy with the continuous water treatment demand; 2) the economical, low

environmental impact disposal of residues (eg. brines for solar desalination and chemical sludge for

solar Fenton). For the first issue, either energy storage or water storage measures should be taken,

which may involves much higher investment cost (such as batteries for PV electricity). The disposal

of residues can account for a large portion of the water cost and have potentially negative impact on

the environment. Therefore, besides the efforts that aim at improving the efficiency and reducing the

cost of the solar technologies and the water treatment processes, developing novel solutions for these

two issues should also be a focus of future research.

54

Acknowledgement

The authors wish to thank China Scholarship Council and University of Wollongong for their

financial support towards a postgraduate scholarship to the first author.

References

[1] M. Shatat, M. Worall, S. Riffat, Opportunities for solar water desalination worldwide: Review,

Sustainable cities and society, 9 (2013) 67-80.

[2] I.E. Agency, Key world energy statistics, International Energy Agency, 2014.

[3] N. Abas, A. Kalair, N. Khan, Review of fossil fuels and future energy technologies, Futures, 69

(2015) 31-49. [4] J. Blanco, S. Malato, P. Fernández-Ibañez, D. Alarcón, W. Gernjak, M.I. Maldonado, Review of

feasible solar energy applications to water processes, Renewable and Sustainable Energy Reviews,

13 (2009) 1437-1445.

[5] G. Masson, M. Latour, D. Biancardi, Global market outlook for photovoltaics until 2016, European

Photovoltaic Industry Association, 5 (2012).

[6] I.E. Agency, Technology Roadmap: Solar Photovoltaic Energy, in, France, 2014.

[7] S. Manju, N. Sagar, Renewable energy integrated desalination: A sustainable solution to

overcome future fresh-water scarcity in India, Renewable and Sustainable Energy Reviews, 73 (2017)

594-609.

[8] A. Pugsley, A. Zacharopoulos, J.D. Mondol, M. Smyth, Global applicability of solar desalination, Renewable Energy, 88 (2016) 200-219.

[9] I.J. Esfahani, J. Rashidi, P. Ifaei, C. Yoo, Efficient thermal desalination technologies with renewable

energy systems: A state-of-the-art review, Korean Journal of Chemical Engineering, 33 (2016) 351-

387.

[10] H. Sharon, K.S. Reddy, A review of solar energy driven desalination technologies, Renewable and

Sustainable Energy Reviews, 41 (2015) 1080-1118.

[11] J.H. Reif, W. Alhalabi, Solar-thermal powered desalination: Its significant challenges and

potential, Renewable and Sustainable Energy Reviews, 48 (2015) 152-165.

[12] S. Gorjian, B. Ghobadian, Solar desalination: A sustainable solution to water crisis in Iran,

Renewable and Sustainable Energy Reviews, 48 (2015) 571-584. [13] C. Li, Y. Goswami, E. Stefanakos, Solar assisted sea water desalination: A review, Renewable and

Sustainable Energy Reviews, 19 (2013) 136-163.

[14] A. Al-Karaghouli, L.L. Kazmerski, Energy consumption and water production cost of conventional

and renewable-energy-powered desalination processes, Renewable and Sustainable Energy Reviews,

24 (2013) 343-356.

[15] J. Kucera, Desalination: water from water, John Wiley & Sons, 2014.

[16] G.N. Tiwari, H.N. Singh, R. Tripathi, Present status of solar distillation, Solar Energy, 75 (2003)

367-373.

[17] P. Prakash, V. Velmurugan, Parameters influencing the productivity of solar stills – A review,

Renewable and Sustainable Energy Reviews, 49 (2015) 585-609. [18] G. Xiao, X. Wang, M. Ni, F. Wang, W. Zhu, Z. Luo, K. Cen, A review on solar stills for brine

desalination, Applied Energy, 103 (2013) 642-652.

[19] V. Sivakumar, E. Ganapathy Sundaram, Improvement techniques of solar still efficiency: A

review, Renewable and Sustainable Energy Reviews, 28 (2013) 246-264.

[20] T. Rajaseenivasan, K.K. Murugavel, T. Elango, R.S. Hansen, A review of different methods to

enhance the productivity of the multi-effect solar still, Renewable and Sustainable Energy Reviews,

17 (2013) 248-259.

55

[21] G.M. Ayoub, L. Malaeb, Developments in solar still desalination systems: A critical review,

Critical Reviews in Environmental Science and Technology, 42 (2012) 2078-2112.

[22] V. Velmurugan, K. Srithar, Performance analysis of solar stills based on various factors affecting

the productivity—A review, Renewable and Sustainable Energy Reviews, 15 (2011) 1294-1304.

[23] A.E. Kabeel, S.A. El-Agouz, Review of researches and developments on solar stills, Desalination, 276 (2011) 1-12.

[24] A. Ahsan, M. Imteaz, U.A. Thomas, M. Azmi, A. Rahman, N.N. Nik Daud, Parameters affecting

the performance of a low cost solar still, Applied Energy, 114 (2014) 924-930.

[25] T. Elango, A. Kannan, K.K. Murugavel, Performance study on single basin single slope solar still

with different water nanofluids, Desalination, 360 (2015) 45-51.

[26] A.E. Kabeel, Z.M. Omara, F.A. Essa, Enhancement of modified solar still integrated with external

condenser using nanofluids: An experimental approach, Energy Conversion and Management, 78

(2014) 493-498.

[27] A. Ahsan, M. Imteaz, A. Rahman, B. Yusuf, T. Fukuhara, Design, fabrication and performance

analysis of an improved solar still, Desalination, 292 (2012) 105-112. [28] G.M. Ayoub, L. Malaeb, Economic feasibility of a solar still desalination system with enhanced

productivity, Desalination, 335 (2014) 27-32.

[29] R.S. Hansen, C.S. Narayanan, K.K. Murugavel, Performance analysis on inclined solar still with

different new wick materials and wire mesh, Desalination, 358 (2015) 1-8.

[30] Y.A.F. El-Samadony, A.S. Abdullah, Z.M. Omara, Experimental study of stepped solar still

integrated with reflectors and external condenser, Experimental Heat Transfer, 28 (2015) 392-404.

[31] M. El-Naggar, A.A. El-Sebaii, M.R.I. Ramadan, S. Aboul-Enein, Experimental and theoretical

performance of finned-single effect solar still, Desalination and Water Treatment, (2015) 1-16.

[32] S.A. El-Agouz, Experimental investigation of stepped solar still with continuous water circulation,

Energy Conversion and Management, 86 (2014) 186-193. [33] T. Rajaseenivasan, K. Kalidasa Murugavel, T. Elango, Performance and exergy analysis of a

double-basin solar still with different materials in basin, Desalination and Water Treatment, (2014)

1-9.

[34] Z.M. Omara, A.E. Kabeel, M.M. Younes, Enhancing the stepped solar still performance using

internal reflectors, Desalination, 314 (2013) 67-72.

[35] F.F. Tabrizi, M. Dashtban, H. Moghaddam, Experimental investigation of a weir-type cascade

solar still with built-in latent heat thermal energy storage system, Desalination, 260 (2010) 248-253.

[36] A.A. El-Sebaii, A.A. Al-Ghamdi, F.S. Al-Hazmi, A.S. Faidah, Thermal performance of a single basin

solar still with PCM as a storage medium, Applied Energy, 86 (2009) 1187-1195.

[37] S. Kumar, G.N. Tiwari, Life cycle cost analysis of single slope hybrid (PV/T) active solar still, Applied Energy, 86 (2009) 1995-2004.

[38] A.E. Kabeel, Performance of solar still with a concave wick evaporation surface, Energy, 34

(2009) 1504-1509.

[39] S. Abdallah, O.O. Badran, Sun tracking system for productivity enhancement of solar still,

Desalination, 220 (2008) 669-676.

[40] Z.S. Abdel-Rehim, A. Lasheen, Experimental and theoretical study of a solar desalination system

located in Cairo, Egypt, Desalination, 217 (2007) 52-64.

[41] H.E.S. Fath, M. El-Samanoudy, K. Fahmy, A. Hassabou, Thermal-economic analysis and

comparison between pyramid-shaped and single-slope solar still configurations, Desalination, 159

(2003) 69-79. [42] E.E. Delyannis, V. Belessiotis, Solar application in desalination: the Greek Islands experiment,

Desalination, 100 (1995) 27-34.

[43] G.N. Tiwari, Demonstration plant of a multi wick solar still, Energy Conversion and Management,

24 (1984) 313-316.

56

[44] F.A. Al-Sulaiman, M.I. Zubair, M. Atif, P. Gandhidasan, S.A. Al-Dini, M.A. Antar, Humidification

dehumidification desalination system using parabolic trough solar air collector, Applied Thermal

Engineering, 75 (2015) 809-816.

[45] B. Seifert, A. Kroiss, M. Spinnler, T. Sattelmayer, About the history of humidification–

dehumidification desalination systems, in, 2013. [46] W. Abdelmoez, M.S. Mahmoud, T.E. Farrag, Water desalination using

humidification/dehumidification (HDH) technique powered by solar energy: a detailed review,

Desalination and Water Treatment, 52 (2014) 4622-4640.

[47] A. Giwa, N. Akther, A.A. Housani, S. Haris, S.W. Hasan, Recent advances in humidification

dehumidification (HDH) desalination processes: Improved designs and productivity, Renewable and

Sustainable Energy Reviews, 57 (2016) 929-944.

[48] M. Zamen, S.M. Soufari, S.A. Vahdat, M. Amidpour, M.A. Zeinali, H. Izanloo, H. Aghababaie,

Experimental investigation of a two-stage solar humidification–dehumidification desalination

process, Desalination, 332 (2014) 1-6.

[49] G.P. Narayan, M.H. Sharqawy, E.K. Summers, J.H. Lienhard, S.M. Zubair, M.A. Antar, The potential of solar-driven humidification–dehumidification desalination for small-scale decentralized

water production, Renewable and Sustainable Energy Reviews, 14 (2010) 1187-1201.

[50] X. Li, G. Yuan, Z. Wang, H. Li, Z. Xu, Experimental study on a humidification and dehumidification

desalination system of solar air heater with evacuated tubes, Desalination, 351 (2014) 1-8.

[51] E.K. Summers, M.A. Antar, J.H. Lienhard V, Design and optimization of an air heating solar

collector with integrated phase change material energy storage for use in humidification–

dehumidification desalination, Solar Energy, 86 (2012) 3417-3429.

[52] E.K. Summers, J.H. Lienhard, S.M. Zubair, Air-heating solar collectors for humidification-

dehumidification desalination systems, Journal of Solar Energy Engineering, 133 (2011) 011016.

[53] E.K. Summers, High efficiency solar air heaters with novel built-in heat storage for use in a humidification-dehumidification desalination cycle, (2010).

[54] S.M. Soufari, M. Zamen, M. Amidpour, Experimental validation of an optimized solar

humidification-dehumidification desalination unit, Desalination and Water Treatment, 6 (2009) 244-

251.

[55] K. Zhani, H. Ben Bacha, Experimental investigation of a new solar desalination prototype using

the humidification dehumidification principle, Renewable Energy, 35 (2010) 2610-2617.

[56] A.M.A. Dayem, Efficient Solar Desalination System Using Humidification/Dehumidification

Process, Journal of Solar Energy Engineering, 136 (2014) 041014.

[57] A.M.A. Dayem, Pioneer solar water desalination system: Experimental testing and numerical

simulation, Energy Science and Technology, 1 (2011) 33-48. [58] A.E. Kabeel, E.M.S. El-Said, A hybrid solar desalination system of air humidification,

dehumidification and water flashing evaporation: Part II. Experimental investigation, Desalination,

341 (2014) 50-60.

[59] Z. Chang, H. Zheng, Y. Yang, Y. Su, Z. Duan, Experimental investigation of a novel multi-effect

solar desalination system based on humidification–dehumidification process, Renewable Energy, 69

(2014) 253-259.

[60] G. Yuan, Z. Wang, H. Li, X. Li, Experimental study of a solar desalination system based on

humidification–dehumidification process, Desalination, 277 (2011) 92-98.

[61] E. Mathioulakis, G. Panaras, V. Belessiotis, Experience gained through the implementation and

operation of a solar humidification—dehumidification desalination plant, Desalination and water treatment, 21 (2010) 375-381.

[62] C. Yamalı, İ. Solmus, A solar desalination system using humidification–dehumidification process:

experimental study and comparison with the theoretical results, Desalination, 220 (2008) 538-551.

[63] I. Houcine, M. BenAmara, A. Guizani, M. Maâlej, Pilot plant testing of a new solar desalination

process by a multiple-effect-humidification technique, Desalination, 196 (2006) 105-124.

57

[64] S. Burn, M. Hoang, D. Zarzo, F. Olewniak, E. Campos, B. Bolto, O. Barron, Desalination

techniques — A review of the opportunities for desalination in agriculture, Desalination, 364 (2015)

2-16.

[65] M.T. Ali, H.E.S. Fath, P.R. Armstrong, A comprehensive techno-economical review of indirect

solar desalination, Renewable and Sustainable Energy Reviews, 15 (2011) 4187-4199. [66] S.M.A. Moustafa, D.I. Jarrar, H.I. El-Mansy, Performance of a self-regulating solar multistage

flash desalination system, Solar Energy, 35 (1985) 333-340.

[67] M.A. Sharaf Eldean, H.E. Fath, Exergy and thermo-economic analysis of solar thermal cycles

powered multi-stage flash desalination process, Desalination and Water Treatment, 51 (2013) 7361-

7378.

[68] A.S. Nafey, M.A. Mohamad, S.O. El-Helaby, M.A. Sharaf, Theoretical and experimental study of a

small unit for solar desalination using flashing process, Energy Conversion and Management, 48

(2007) 528-538.

[69] H. Lu, J.C. Walton, H. Hein, Thermal desalination using MEMS and salinity-gradient solar pond

technology, NASA STI/Recon Technical Report N, 3 (2002) 00498. [70] A.S. Nafey, M.A. Sharaf, L. García-Rodríguez, A new visual library for design and simulation of

solar desalination systems (SDS), Desalination, 259 (2010) 197-207.

[71] P. Palenzuela, A.S. Hassan, G. Zaragoza, D.-C. Alarcón-Padilla, Steady state model for multi-

effect distillation case study: Plataforma Solar de Almería MED pilot plant, Desalination, 337 (2014)

31-42.

[72] M. Al-Shammiri, M. Safar, Multi-effect distillation plants: state of the art, Desalination, 126

(1999) 45-59.

[73] M.A. Sharaf, A.S. Nafey, L. García-Rodríguez, Thermo-economic analysis of solar thermal power

cycles assisted MED-VC (multi effect distillation-vapor compression) desalination processes, Energy,

36 (2011) 2753-2764. [74] D.-C. Alarcón-Padilla, L. García-Rodríguez, Application of absorption heat pumps to multi-effect

distillation: a case study of solar desalination, Desalination, 212 (2007) 294-302.

[75] M.D. Stuber, C. Sullivan, S.A. Kirk, J.A. Farrand, P.V. Schillaci, B.D. Fojtasek, A.H. Mandell, Pilot

demonstration of concentrated solar-powered desalination of subsurface agricultural drainage

water and other brackish groundwater sources, Desalination, 355 (2015) 186-196.

[76] D.-C. Alarcón-Padilla, J. Blanco-Gálvez, L. García-Rodríguezz, W. Gernjak, S. Malato-Rodríguez,

First experimental results of a new hybrid solar/gas multi-effect distillation system: the AQUASOL

project, Desalination, 220 (2008) 619-625.

[77] D.-C. Alarcón-Padilla, L. García-Rodríguez, J. Blanco-Gálvez, Assessment of an absorption heat

pump coupled to a multi-effect distillation unit within AQUASOL project, Desalination, 212 (2007) 303-310.

[78] B. Milow, E. Zarza, Advanced MED solar desalination plants. Configurations, costs, future—

seven years of experience at the Plataforma Solar de Almeria (Spain), Desalination, 108 (1997) 51-58.

[79] P. Palenzuela, G. Zaragoza, D.-C. Alarcón-Padilla, Characterisation of the coupling of multi-effect

distillation plants to concentrating solar power plants, Energy, 82 (2015) 986-995.

[80] S. Casimiro, J. Cardoso, C. Ioakimidis, J.F. Mendes, C. Mineo, A. Cipollina, MED parallel system

powered by concentrating solar power (CSP). Model and case study: Trapani, Sicily, (2014).

[81] R. Olwig, T. Hirsch, C. Sattler, H. Glade, L. Schmeken, S. Will, A. Ghermandi, R. Messalem,

Techno-economic analysis of combined concentrating solar power and desalination plant

configurations in Israel and Jordan, Desalination and Water Treatment, 41 (2012) 9-25. [82] M.A. Sharaf, A.S. Nafey, L. García-Rodríguez, Exergy and thermo-economic analyses of a

combined solar organic cycle with multi effect distillation (MED) desalination process, Desalination,

272 (2011) 135-147.

[83] N. Ghaffour, T.M. Missimer, G.L. Amy, Technical review and evaluation of the economics of

water desalination: Current and future challenges for better water supply sustainability, Desalination,

309 (2013) 197-207.

58

[84] X. Liu, W. Chen, M. Gu, S. Shen, G. Cao, Thermal and economic analyses of solar desalination

system with evacuated tube collectors, Solar Energy, 93 (2013) 144-150.

[85] Y.-D. Kim, K. Thu, A. Myat, K.C. Ng, Numerical simulation of solar-assisted multi-effect

distillation (SMED) desalination systems, Desalination and Water Treatment, 51 (2013) 1242-1253.

[86] J. Leblanc, J. Andrews, A. Akbarzadeh, Low‐temperature solar–thermal multi‐effect

evaporation desalination systems, International Journal of Energy Research, 34 (2010) 393-403.

[87] D.C. Alarcón-Padilla, L. García-Rodríguez, J. Blanco-Gálvez, Design recommendations for a multi-

effect distillation plant connected to a double-effect absorption heat pump: A solar desalination case

study, Desalination, 262 (2010) 11-14.

[88] A. Alkhudhiri, N. Darwish, N. Hilal, Membrane distillation: A comprehensive review, Desalination,

287 (2012) 2-18.

[89] W. Heinzl, S. Büttner, G. Lange, Industrialized modules for MED Desalination with polymer

surfaces, Desalination and Water Treatment, 42 (2012) 177-180.

[90] K. Zhao, W. Heinzl, M. Wenzel, S. Büttner, F. Bollen, G. Lange, S. Heinzl, N. Sarda, Experimental

study of the memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module, Desalination, 323 (2013) 150-160.

[91] E. Drioli, A. Ali, F. Macedonio, Membrane distillation: Recent developments and perspectives,

Desalination, 356 (2015) 56-84.

[92] M. Essalhi, M. Khayet, Application of a porous composite hydrophobic/hydrophilic membrane in

desalination by air gap and liquid gap membrane distillation: A comparative study, Separation and

Purification Technology, 133 (2014) 176-186.

[93] G. Naidu, S. Jeong, Y. Choi, E. Jang, T.-M. Hwang, S. Vigneswaran, Application of vacuum

membrane distillation for small scale drinking water production, Desalination, 354 (2014) 53-61.

[94] F. Banat, N. Jwaied, Autonomous membrane distillation pilot plant unit driven solar energy:

Experiences and lessons learned, Int. J. Sustain. Water Environ. Syst, 1 (2010) 21-24. [95] J. Koschikowski, M. Wieghaus, M. Rommel, V.S. Ortin, B.P. Suarez, J.R. Betancort Rodríguez,

Experimental investigations on solar driven stand-alone membrane distillation systems for remote

areas, Desalination, 248 (2009) 125-131.

[96] H.E.S. Fath, S.M. Elsherbiny, A.A. Hassan, M. Rommel, M. Wieghaus, J. Koschikowski, M.

Vatansever, PV and thermally driven small-scale, stand-alone solar desalination systems with very

low maintenance needs, Desalination, 225 (2008) 58-69.

[97] F. Banat, N. Jwaied, M. Rommel, J. Koschikowski, M. Wieghaus, Performance evaluation of the

“large SMADES” autonomous desalination solar-driven membrane distillation plant in Aqaba, Jordan,

Desalination, 217 (2007) 17-28.

[98] F. Banat, N. Jwaied, M. Rommel, J. Koschikowski, M. Wieghaus, Desalination by a “compact SMADES” autonomous solarpowered membrane distillation unit, Desalination, 217 (2007) 29-37.

[99] A. Chafidz, S. Al-Zahrani, M.N. Al-Otaibi, C.F. Hoong, T.F. Lai, M. Prabu, Portable and integrated

solar-driven desalination system using membrane distillation for arid remote areas in Saudi Arabia,

Desalination, 345 (2014) 36-49.

[100] R.G. Raluy, R. Schwantes, V.J. Subiela, B. Peñate, G. Melián, J.R. Betancort, Operational

experience of a solar membrane distillation demonstration plant in Pozo Izquierdo-Gran Canaria

Island (Spain), Desalination, 290 (2012) 1-13.

[101] F. Banat, N. Jwaied, Economic evaluation of desalination by small-scale autonomous solar-

powered membrane distillation units, Desalination, 220 (2008) 566-573. [102] R.B. Saffarini, E.K. Summers, H.A. Arafat, J.H. Lienhard V, Economic evaluation of stand-alone

solar powered membrane distillation systems, Desalination, 299 (2012) 55-62.

[103] W.G. Shim, K. He, S. Gray, I.S. Moon, Solar energy assisted direct contact membrane distillation

(DCMD) process for seawater desalination, Separation and Purification Technology, 143 (2015) 94-

104.

59

[104] G. Zaragoza, A. Ruiz-Aguirre, E. Guillén-Burrieza, Efficiency in the use of solar thermal energy of

small membrane desalination systems for decentralized water production, Applied Energy, 130

(2014) 491-499.

[105] Y. Wang, Z. Xu, N. Lior, H. Zeng, An experimental study of solar thermal vacuum membrane

distillation desalination, Desalination and Water Treatment, (2014) 1-11. [106] Y.-D. Kim, K. Thu, N. Ghaffour, K. Choon Ng, Performance investigation of a solar-assisted

direct contact membrane distillation system, Journal of Membrane Science, 427 (2013) 345-364.

[107] S. Gabsi, N. Frikha, B. Chaouachi, Performance of a Solar Vacuum Membrane Distillation Pilot

Plant, for Seawater Desalination in Mahares, Tunisia, (2013).

[108] N. Frikha, R. Matlaya, B. Chaouachi, S. Gabsi, Simulation of an autonomous solar vacuum

membrane distillation for seawater desalination, Desalination and Water Treatment, 52 (2013)

1725-1734.

[109] S. Ben Abdallah, N. Frikha, S. Gabsi, Design of an autonomous solar desalination plant using

vacuum membrane distillation, the MEDINA project, Chemical Engineering Research and Design, 91

(2013) 2782-2788. [110] E. Guillén-Burrieza, J. Blanco, G. Zaragoza, D.-C. Alarcón, P. Palenzuela, M. Ibarra, W. Gernjak,

Experimental analysis of an air gap membrane distillation solar desalination pilot system, Journal of

Membrane Science, 379 (2011) 386-396.

[111] N. Dow, M. Duke, J. Zhang, T. O’Rielly, J.D. Li, S. Gray, E. Ostarcevic, P. Atherton,

Demonstration of solar driven membrane distillation in remote Victoria, in, 2010, pp. 8-10.

[112] X. Wang, L. Zhang, H. Yang, H. Chen, Feasibility research of potable water production via solar-

heated hollow fiber membrane distillation system, Desalination, 247 (2009) 403-411.

[113] A. Al-Karaghouli, D. Renne, L.L. Kazmerski, Technical and economic assessment of

photovoltaic-driven desalination systems, Renewable Energy, 35 (2010) 323-328.

[114] B. Peñate, V.J. Subiela, F. Vega, F. Castellano, F.J. Domínguez, V. Millán, Uninterrupted eight-year operation of the autonomous solar photovoltaic reverse osmosis system in Ksar Ghilène

(Tunisia), Desalination and Water Treatment, (2014) 1-8.

[115] H. Qiblawey, F. Banat, Q. Al-Nasser, Performance of reverse osmosis pilot plant powered by

Photovoltaic in Jordan, Renewable Energy, 36 (2011) 3452-3460.

[116] F. Banat, H. Qiblawey, Q. Al-Nasser, Economic evaluation of a small RO unit powered by PV

installed in the village of Hartha, Jordan, Desalination and Water Treatment, 3 (2009) 169-174.

[117] A. Alsheghri, S.A. Sharief, S. Rabbani, N.Z. Aitzhan, Design and Cost Analysis of a Solar

Photovoltaic Powered Reverse Osmosis Plant for Masdar Institute, Energy Procedia, 75 (2015) 319-

324.

[118] A.M. Delgado-Torres, L. García-Rodríguez, Design recommendations for solar organic Rankine cycle (ORC)–powered reverse osmosis (RO) desalination, Renewable and Sustainable Energy Reviews,

16 (2012) 44-53.

[119] G. Xia, Q. Sun, J. Wang, X. Cao, Y. Yu, L. Wang, Theoretical analysis of a reverse osmosis

desalination system driven by solar-powered organic Rankine cycle and wind energy, Desalination

and Water Treatment, 53 (2015) 876-886.

[120] R. Salcedo, E. Antipova, D. Boer, L. Jiménez, G. Guillén-Gosálbez, Multi-objective optimization

of solar Rankine cycles coupled with reverse osmosis desalination considering economic and life

cycle environmental concerns, Desalination, 286 (2012) 358-371.

[121] M. Khayet, M. Essalhi, C. Armenta-Déu, C. Cojocaru, N. Hilal, Optimization of solar-powered

reverse osmosis desalination pilot plant using response surface methodology, Desalination, 261 (2010) 284-292.

[122] A. De Munari, D.P.S. Capão, B.S. Richards, A.I. Schäfer, Application of solar-powered

desalination in a remote town in South Australia, Desalination, 248 (2009) 72-82.

[123] B. Peñate, L. García-Rodríguez, Seawater reverse osmosis desalination driven by a solar

Organic Rankine Cycle: Design and technology assessment for medium capacity range, Desalination,

284 (2012) 86-91.

60

[124] N.C. Wright, A.G. Winter V, Justification for community-scale photovoltaic-powered

electrodialysis desalination systems for inland rural villages in India, Desalination, 352 (2014) 82-91.

[125] B. Peñate, F. Círez, F.J. Domínguez, V.J. Subiela, L. Vera, Design and testing of an isolated

commercial EDR plant driven by solar photovoltaic energy, Desalination and Water Treatment, 51

(2013) 1254-1264. [126] Y. Zhang, L. Pinoy, B. Meesschaert, B. Van der Bruggen, A natural driven membrane process for

brackish and wastewater treatment: photovoltaic powered ED and FO hybrid system, Environmental

science & technology, 47 (2013) 10548-10555.

[127] J.M. Ortiz, E. Expósito, F. Gallud, V. García-García, V. Montiel, A. Aldaz, Desalination of

underground brackish waters using an electrodialysis system powered directly by photovoltaic

energy, Solar Energy Materials and Solar Cells, 92 (2008) 1677-1688.

[128] C. Fernandez-Gonzalez, A. Dominguez-Ramos, R. Ibañez, A. Irabien, Sustainability assessment

of electrodialysis powered by photovoltaic solar energy for freshwater production, Renewable and

Sustainable Energy Reviews, 47 (2015) 604-615.

[129] F. Círez, J. Uche, A.A. Bayod, A. Martínez, Batch ED fed by a PV unit: a reliable, flexible, and sustainable integration, Desalination and Water Treatment, 51 (2013) 673-685.

[130] H.M.N. AlMadani, Water desalination by solar powered electrodialysis process, Renewable

Energy, 28 (2003) 1915-1924.

[131] I.C. Karagiannis, P.G. Soldatos, Water desalination cost literature: review and assessment,

Desalination, 223 (2008) 448-456.

[132] M. Papapetrou, C. Biercamp, M. Wieghaus, Roadmap for the Development of Desalination

Powered by Renewable Energy: Promotion for Renewable Energy for Water Production Through

Desalination, Fraunhofer Verlag, 2010.

[133] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water

treatment technology: A review, Water Research, 44 (2010) 2997-3027. [134] S. Ahmed, M.G. Rasul, W.N. Martens, R. Brown, M.A. Hashib, Heterogeneous photocatalytic

degradation of phenols in wastewater: A review on current status and developments, Desalination,

261 (2010) 3-18.

[135] D. Spasiano, R. Marotta, S. Malato, P. Fernandez-Ibañez, I. Di Somma, Solar photocatalysis:

Materials, reactors, some commercial, and pre-industrialized applications. A comprehensive

approach, Applied Catalysis B: Environmental, 170–171 (2015) 90-123.

[136] R.L. Pozzo, J.L. Giombi, M.A. Baltanás, A.E. Cassano, The performance in a fluidized bed reactor

of photocatalysts immobilized onto inert supports, Catalysis Today, 62 (2000) 175-187.

[137] S. Rahim Pouran, A.R. Abdul Aziz, W.M.A. Wan Daud, Review on the main advances in photo-

Fenton oxidation system for recalcitrant wastewaters, Journal of Industrial and Engineering Chemistry, 21 (2015) 53-69.

[138] R.J. Braham, A.T. Harris, Review of Major Design and Scale-up Considerations for Solar

Photocatalytic Reactors, Industrial & Engineering Chemistry Research, 48 (2009) 8890-8905.

[139] D. Bahnemann, Photocatalytic water treatment: solar energy applications, Solar Energy, 77

(2004) 445-459.

[140] M.A. Sousa, C. Gonçalves, V.J.P. Vilar, R.A.R. Boaventura, M.F. Alpendurada, Suspended TiO2-

assisted photocatalytic degradation of emerging contaminants in a municipal WWTP effluent using a

solar pilot plant with CPCs, Chemical Engineering Journal, 198–199 (2012) 301-309.

[141] N. Miranda-García, S. Suárez, B. Sánchez, J.M. Coronado, S. Malato, M.I. Maldonado,

Photocatalytic degradation of emerging contaminants in municipal wastewater treatment plant effluents using immobilized TiO2 in a solar pilot plant, Applied Catalysis B: Environmental, 103 (2011)

294-301.

[142] S. Malato, P. Fernández-Ibáñez, M.I. Maldonado, J. Blanco, W. Gernjak, Decontamination and

disinfection of water by solar photocatalysis: Recent overview and trends, Catalysis Today, 147 (2009)

1-59.

61

[143] I. Michael, A. Panagi, L.A. Ioannou, Z. Frontistis, D. Fatta-Kassinos, Utilizing solar energy for the

purification of olive mill wastewater using a pilot-scale photocatalytic reactor after coagulation-

flocculation, Water Research, 60 (2014) 28-40.

[144] J. Fenoll, P. Flores, P. Hellín, C.M. Martínez, S. Navarro, Photodegradation of eight

miscellaneous pesticides in drinking water after treatment with semiconductor materials under sunlight at pilot plant scale, Chemical Engineering Journal, 204–206 (2012) 54-64.

[145] G. Zayani, L. Bousselmi, F. Mhenni, A. Ghrabi, Solar photocatalytic degradation of commercial

textile azo dyes: Performance of pilot plant scale thin film fixed-bed reactor, Desalination, 246 (2009)

344-352.

[146] T. Velegraki, D. Mantzavinos, Solar photo-Fenton treatment of winery effluents in a pilot

photocatalytic reactor, Catalysis Today, 240, Part A (2015) 153-159.

[147] M.J. Hernández-Rodríguez, C. Fernández-Rodríguez, J.M. Doña-Rodríguez, O.M. González-Díaz,

D. Zerbani, J. Pérez Peña, Treatment of effluents from wool dyeing process by photo-Fenton at solar

pilot plant, Journal of Environmental Chemical Engineering, 2 (2014) 163-171.

[148] M.B. Keogh, M. Castro-Alférez, M.I. Polo-López, I. Fernández Calderero, Y.A. Al-Eryani, C. Joseph-Titus, B. Sawant, R. Dhodapkar, C. Mathur, K.G. McGuigan, P. Fernández-Ibáñez, Capability of

19-L polycarbonate plastic water cooler containers for efficient solar water disinfection (SODIS):

Field case studies in India, Bahrain and Spain, Solar Energy, 116 (2015) 1-11.

[149] K. McGuigan, T. Joyce, R. Conroy, J. Gillespie, M. Elmore-Meegan, Solar disinfection of drinking

water contained in transparent plastic bottles: characterizing the bacterial inactivation process,

Journal of applied microbiology, 84 (1998) 1138-1148.

[150] K.G. McGuigan, R.M. Conroy, H.-J. Mosler, M.d. Preez, E. Ubomba-Jaswa, P. Fernandez-Ibañez,

Solar water disinfection (SODIS): A review from bench-top to roof-top, Journal of Hazardous

Materials, 235–236 (2012) 29-46.

[151] M. Vivar, M. Fuentes, J. Castro, R. García-Pacheco, Effect of common rooftop materials as support base for solar disinfection (SODIS) in rural areas under temperate climates, Solar Energy,

115 (2015) 204-216.

[152] T. Clasen, S. Cairncross, L. Haller, J. Bartram, D. Walker, Cost-effectiveness of water quality

interventions for preventing diarrhoeal disease in developing countries, Journal of water and health,

5 (2007) 599-608.